Cementum has been shown to contain unique polypeptides that participate in cell recruitment and differentiation during cementum formation. We report the isolation of a cDNA variant for protein-tyrosine phosphatase-like (proline instead of catalytic arginine) member-a (PTPLA) from cementum. A cementifying fibroma-derived λ-ZAP expression library was screened by panning with a monoclonal antibody to cementum attachment protein (CAP), and 1435 bp cDNA (gb AC093525.3) was isolated. This cDNA encodes a 140-amino-acid polypeptide, and its N-terminal 125 amino acids are identical to those of PTPLA. This isoform, designated as PTPLA-CAP, results from a read-through of the PTPLA exon 2 splice donor site, truncating after the second putative transmembrane domain. It contains 15 amino acids encoded within the intron between PTPLA exons 2 and 3, which replace the active site for PTPLA phosphatase activity. The recombinant protein, rhPTPLA-CAP, has Mr 19 kDa and cross-reacts with anti-CAP antibody. Anti-rhPTPLA-CAP antibody immunostained cementum cells, cementum, heart, and liver. Quantitative RT-PCR showed that PTPLA was expressed in all periodontal cells; however, PTPLA-CAP expression was limited to cementum cells. The rhPTPLA-CAP promoted gingival fibroblast attachment. We conclude that PTPLA-CAP is a splice variant of PTPLA, and that, in the periodontium, cementum and cementum cells express this variant.