EXPRESSION OF VESICLE-ASSOCIATED MEMBRANE-PROTEIN 2 (VAMP-2) SYNAPTOBREVIN-II AND CELLUBREVIN IN RAT SKELETAL-MUSCLE AND IN A MUSCLE-CELL LINE

Molecular studies have identified a family of synaptic vesicle-associa ted membrane proteins (VAMPs, also known as synaptobrevins) which have been implicated in synaptic vesicle docking and/or fusion with plasma membrane proteins. Here we demonstrate the expression of two members of this family, VAMP-2/synaptobrevin II and cellubrevin, in skeletal m uscle, a tissue with both constitutive and regulated membrane traffic. The 18 kDa VAMP-2 polypeptide was detected in purified membrane fract ions from adult skeletal muscle and from L6 myotubes in culture, demon strating that the presence of this protein in the isolated muscle memb rane fractions is not the result of contamination by ancillary tissues such as peripheral nerve. Furthermore, skeletal muscle and the muscle cell line also expressed cellubrevin, a VAMP-2 homologue of 17 kDa, w hich is much less abundant in brain cells. Both VAMP-2 and cellubrevin were preferentially isolated in membrane fractions rich in plasma mem branes, and were less concentrated in light microsomes and other inter nal membrane fractions of mature muscle or muscle cells in culture. In terestingly, both VAMP-2 and cellubrevin were much more abundant in th e differentiated L6 myotubes than in their precursor myoblasts, sugges ting that they are required for functions of differentiated muscle cel ls. The identity of both polypeptides was further confirmed by their s usceptibility to proteolysis by Clostridium tetanus toxin. Expression of these products was further established by the presence of mRNA tran scripts of VAMP-2 and cellubrevin, but not of VAMP-1, in both skeletal muscle and L6 myotubes. In contrast, other synaptic vesicle and docki ng/fusion components were undetectable, such as VAMP-1, SNAP25 and syn taxin 1A/1B, as were synaptophysin and synapsin Ia/Ib, proteins which are believed to be involved in sensing the signal for neuronal exocyto sis. It is concluded that VAMP-2 and cellubrevin are expressed in skel etal muscle cells and may each participate in specific processes of in tracellular membrane traffic.