Molecular Cloning, Expression, and Chromosomal Assignment of Sarcolemmal-associated Proteins

Two overlapping cDNAs encoding a novel sarcolemmal associated protein (SLAP) were isolated from a cardiac cDNA expression library by immunoscreening with anti-sarcolemmal antibodies. Further characterization of these clones showed that they belonged to a family of related cDNAs that potentially encode polypeptides of 37, 46, and 74 kDa designated SLAP1, SLAP2, and SLAP3, respectively. The SLAP3 transcript was ubiquitously expressed, whereas SLAP1 and SLAP2 transcripts were predominantly expressed in cardiac, soleus, and smooth muscle. SLAP was encoded by a single gene that mapped to chromosome 3p14.3–21.2, and the various transcripts are likely generated by alternative splicing. The primary structure of SLAP predicted that it would have large regions of coiled-coil structure including an 11-heptad acidic amphipathic α-helical segment. The carboxyl-terminal region of the SLAP proteins was predicted to have a transmembrane domain, although there was no discernible signal sequence. SLAPs could only be solubilized from cardiac membrane with detergents suggesting that they were integral membrane proteins. Subcellular distribution studies showed that MYC epitope-tagged SLAP localized to regions of juxtaposition between neighboring cell membranes although an intracellular pool of the protein was also present in cells undergoing apparent cleavage. Immunohistochemical localization of SLAP in cardiac muscle revealed that SLAP associated with the sarcolemma and also displayed a reticular pattern of staining that resembled the transverse tubules and the sarcoplasmic reticulum. The SLAPs define a new family of tail-anchored membrane proteins that exhibit tissue-specific expression and are uniquely situated to serve a variety of roles through their coiled-coil motifs.