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A 60-kDa protein localised in adherens-type cellular junctions, and previously called aciculin, has been found to interact with the cytoskeletal proteins dystrophin and utrophin [Belkin, A. M. & Burridge, K. (1995) J. Biol. Chem. 270, 6328–6337]. In this study, we report the complete sequence of this protein, and show that it is a novel member of the phosphoglucomutase (PGM) family of proteins. The PGM-related protein (PGM-RP), which contains 506 amino acids (55.6 kDa), is smaller than PGM1 (566 amino acids, 61 kDa). The active site consensus sequences of prokaryotic and eukaryotic mutases are not conserved in PGM-RP, a finding consistent with the lack of enzymatic activity of PGM-RP in vitro, and the absence of a phosphorylated intermediate in vivo. The organisation of the PGM-RP gene is essentially identical to that of PGM1. We propose that the PGM-RP gene, which we have mapped to human chromosome 9qcen-q13, evolved from the PGM1 gene, and encodes a protein with a structural rather than an enzymatic role. PGM-RP is expressed predominantly in muscle with the highest levels in smooth muscle. The significance of the interaction between dystrophin/utrophin and an increasing number of cytoplasmic proteins including PGM-RP remains to be explored.

We have recently characterised a 60-kDa muscle-specific phosphoglucomutase-related protein (PGM-RP) which is expressed predominantly in adult visceral and vascular smooth muscle. Here we show that the adult vascular smooth muscle cell line PAC1, which retains the capacity to synthesise metavinculin (a marker of the contractile phenotype) also expressed PGM-RP. However, an embryonic smooth muscle cell line A10, which lacks metavinculin, expressed low levels of PGM-RP. Levels of PGM-RP increased in quiescent PAC1 and A10 cells, and were elevated in response to angiotensin II. PGM-RP is therefore a good marker of the contractile/differentiated smooth muscle phenotype. We have sequenced 1.8 kb of the human PGM-RP promoter and shown that it lacks a conventional TATA box. There are multiple transcription start sites, the most predominant of which are inside an initiator sequence (Inr), which is close to two CT boxes and a GATA element. A minimal promoter-CAT construct (p57-CAT) containing the Inr, a CT box and GATA element directed high-level chloramphenicol acetyltransferase (CAT) expression in the differentiated smooth muscle cell line PAC1, and low-level expression in the embryonic smooth muscle cell line A10. This fits well with the pattern of expression of the endogenous gene. A construct (p146-CAT) containing all of the mRNA initiation sites directed a reduced level of CAT expression, and constructs containing 1.8 kb and 3.3 kb upstream of the major transcription start site displayed even lower activity. Sequence comparisons suggest that the PGM-RP promoter evolved from the main phosphoglucomutase promoter which is active in wide range of cell types. The PGM-RP promoter may have acquired negative regulatory elements as expression of the gene became muscle-specific.