A MYELIN PROTEIN IS ENCODED BY THE HOMOLOG OF A GROWTH ARREST-SPECIFIC GENE

Striking features of the cellular response to sciatic nerve injury are the proliferation of Schwann cells in the distal nerve stump and the downregulation of myelin-specific gene expression. Once the axons regrow, the Schwann cells differentiate again to reform the myelin sheaths. We have isolated a rat cDNA, SR13, which is strongly downregulated in the initial phase after sciatic nerve injury. This cDNA encodes a glycoprotein that shares striking amino acid similarity with a purified myelin protein and is specifically precipitated by a myelin-specific antiserum. Immunohistochemistry experiments using peptide-specific polyclonal antibodies localize the SR13 protein to the myelin sheath of the sciatic nerve. Computer-aided sequence analysis identified a pronounced homology of SR13 to a growth arrest-specific mRNA (Gas-3) that is expressed in resting but not in proliferating 3T3 mouse fibroblasts. SR13 is similarly downregulated during Schwann cell proliferation in the rat sciatic nerve. The association of the SR13 as well as the Gas-3 mRNA with nonproliferating cells in two different experimental systems suggests a common role for these molecules in maintaining the quiescent cell state.