The intron/exon structure of the CPM gene was determined from screening human kidney and placenta cDNA libraries (Bektas et al., 2001). The CPM gene contains 11 exons and spans 112.5 kb. The coding region is located in exons 2-9.

Transcription is initiated from multiple transcription start sites clustered in two distinct regions that are flanked by two independent, functional promoters. The proximal promoter (~ 350 bp upstream of the coding region) is characterized by the presence of CpG islands, a classical TATA box (25 bp upstream of the major initiation site), an initiator sequence (Inr) around the TATA box, and a putative downstream promoter element (DPE). A number of potential transcription factor binding sites were identified in the 5 region flanking the proximal initiation sites, including a vitamin D3 responsive element and Sp1. The distal promoter (~ 30 kb upstream of the coding region) differs from the proximal promoter in that it mainly consists of repetitive elements and lacks common promoter elements. Inr sequences and a putative DPE were found together with putative Ets, C/EBP, Oct-1, AP-1 and NF-kB sites. Basal transcriptional activity of the proximal and distal promoter regions was cell type-dependent pointing towards a tissue-specific expression of CPM. Transcriptional initiation from the distal start site appeared less common (Li et al., 2002).
Apart from the full length CPM mRNA, three alternatively splice variants of CPM were detected. Missing exon 3 and/or 5, these products lead to a premature stop codon and possibly to the generation of truncated CPM proteins (Pessoa et al., 2002). Several bands ranging from about 2.4 kb to 15 kb were detected in Northern blots of CPM mRNA from various human tissues, with a major band at 4.2 kb (Tan et al., 1989; Nagae et al., 1993). Heterogeneity in the CPM mRNA was observed, principally ensuing from the 3 region. Together with alternative splicing of three separate exons (1, 1A and 1B), the utilization of various transcription start sites contributes to heterogeneity at the 5 region. 5 and 3 heterogeneity however did not change the CPM protein sequence (Li et al., 2002).
The Ensemble database (viewed June 2013) lists 13 transcripts for CPM of which 3 are coding for the full-length protein and 4 are coding for shorter forms. Of the remainder, there is 1 non-coding processed transcript, 3 are labelled non-sense mediated decay and 2 have a retained intron.

CPM is a basic metallo-carboxypeptidase. The NC-IUBMB code assigned to CPM is EC 3.4.17.12. In the MEROPS database CPM belongs to clan MC, family M14, subfamily B.

The CPM structure consists of two domains, the classical carboxypeptidase domain and the C-terminal domain. The spherical carboxypeptidase domain (first 295 amino acids) is arranged in a typical α/β hydrolase fold and carries the catalytic site. A funnel-shaped entrance gives access to the active site. The C-terminal domain (86 residues) consists of a seven-stranded β-barrel and resembles the plasma protein transthyretin/prealbumin (Reverter et al., 2004). CPM is attached to the outer membrane by a glycosyl-phosphatidyl-inositol (GPI) anchor located at the C-terminus (Deddish et al., 1990).

CPM is widely expressed in the different organs, but expression levels vary and it is only expressed by certain cell types. Expression was studied in some detail in the lung, in the female reproductive system, and in the kidney.
In the lung CPM is a marker of type I pulmonary alveolar epithelial cells (Nagae et al., 1993).
CPM expression is locally regulated during the different phases of the menstrual cycle, endometrial maturation and implantation. Overall, CPM is likely involved in the control of proliferation and functional differentiation of many cellular system within the female reproductive system (Yoshioka et al., 1998; Fujiwara et al., 1999; Fujiwara et al., 2005; Nishioka et al., 2003).
In the kidney, CPM expression is high at the apical surface of proximal and distal tubuli and the thick ascending limbs of the loop of Henle. Soluble CPM was detected in the tubular lumina. CPM was also expressed at the parietal epithelium beneath the Bowmans basement membrane and in glomerular mesangial cells (Denis et al., 2013).
In the central and peripheral nervous systems CPM expression is associated with myelin and myelin-forming cells (Nagae et al., 1992; Kang et al., 2011).
A soluble form of CPM, lacking the membrane anchor, was found in urine, amnion and seminal fluid and in broncho-alveolar lavage fluid.
CPM was at least twice discovered as the target of antibodies raised against cell surface antigens: once on mature macrophages and once on the human B-lineage acute lymphoblastic leukemia cell line Pre ALP. Expression of CPM was reported in late stages of myeloid cell development and in particular stages of B lymphocyte development, i.e. committed precursors and germinal center cells. The expression of CPM in different stages of hematopoietic stem cell differentiation was comprehensively reviewed (Deiteren et al., 2009; Denis et al., 2012). The reader is referred to the publications listed in these reviews; some highlights are repeated below. CPM expression was evident in hematopoietic progenitors (CFU-GM, CFU-Meg and BFU-E). The surface expression of CPM was upregulated during ex vivo expansion of cord blood CD34⁺ stem cells to CFU-GM and CFU-Meg (Marquez-Curtis et al., 2008).
CPM expression is weak on freshly isolated blood monocytes. In contrast to macrophages maturated in vitro, macrophages of body fluids (pleural, peritoneal and alveolar) and tissue macrophages in situ express only low levels of CPM. In defined pathological conditions, some exudate macrophages did express considerable levels of CPM, e.g. alveolar macrophages. Inflammatory macrophages in situ were CPM negative except those associated with rejected renal allografts (Andreesen et al., 1988). CPM is expressed selectively in tissue granulomas and foam cells (Tsakiris et al., 2012) and on tumor associated macrophages (Denis et al., 2013; Tsakiris et al., 2008). Peripheral granulocytes all possessed CPM surface expression. The expression of CPM on several immortalized cell lines was reviewed (Denis and Lambeir, 2013). THP-1 cells, that are close to the mature macrophage, express high levels of CPM.
CPM expression was observed early in mesenchymal differentiation, i.e. in mesenchymal stem cells (MSC) and CFU-F progenitor cells (Marques-Curtis et al., 2008). CPM was upregulated in early and late stages of bone marrow or adipose tissue derived MSC differentiation into the osteogenic, chondrogenic and adipogenic lineages (Lui at al., 2007). CPM expression was greatly increased in early and late stadia of MSC differentiation into the adipocyte and osteogenic lineage compared to the chondrogenic lineage.
Differential transcript analysis identified CPM as a surface marker of heterogeneous peripheral blood-derived smooth muscle progenitor cells (Wang et al., 2012).

The GPI anchor directs CPM to lipid rafts in the outer membrane of cells, such as macrophages. In the kidney CPM is found in the lumen and on the luminal side of epithelial cells in proximal and distal tubules. Intracellular CPM immunoreactivity was also observed (Denis et al., 2013).

The function of CPM in the different cells and organs is not well understood. The expression pattern of CPM in specific cells in the different systems suggests roles in development and/or differentiation. On the one hand CPM may be important for the recycling of amino acids or the local release of arginine. On the other hand, CPM may function by modulating signaling cascades of its substrates (Deiteren et al., 2009). The classical substrates for CPM are anaphylatoxins and kinins, produced during inflammation. However, many other potential substrates have been identified, including hormones, chemokines and growth factors. A functional association of CPM with the bradykinin-1 receptor (a G-protein coupled receptor) has been demonstrated (Zhang et al., 2008; Zhang et al., 2011). CPM enhances bradykinin-1 receptor signaling on two levels: (1) by converting bradykinin to a better agonist (des-arg-bradykinin), and (2) by altering the conformation of the receptor on the membrane. Therefore, one can speculate that the functions of CPM are linked to the functions of bradykinin, e.g. release of inflammatory cytokines, vasodilation and pain.

CPM has significant homology with the M14B subfamily members CPN, CPH/E, CPZ, CPD, CPX-1, CPX-2 and adipocyte enhancer-binding protein 1 (AEBP1).

In the NCBI databases a number of variants can be found in the CPM genomic sequence that were reported in association studies related to blood pressure regulation and heart function (Vasan et al., 2007) and asthma and smoking (Litonjua et al., 2008; Pan et al., 2010). However the clinical relevance of these findings is unknown.