MOLECULAR CHARACTERISTICS OF THE NOVEL INTERMEDIATE FILAMENT PROTEIN PARANEMIN - SEQUENCE REVEALS EAP-300 AND IFAP-400 ARE HIGHLY HOMOLOGOUS TO PARANEMIN

Paranemin was initially found to copurify with the intermediate filame nt (IF) proteins vimentin and desmin from embryonic chick skeletal mus cle and was described as an IF-associated protein (IFAP). We have puri fied paranemin from embryonic chick skeletal muscle, prepared antibodi es, and demonstrated that they label at the Z-lines of both adult avia n and porcine cardiac and skeletal muscle myofibrils. We determined th e cDNA sequence of paranemin by immunoscreening a lambda gt22A cDNA li brary from embryonic chick skeletal muscle. Northern blot analysis rev ealed a single transcript of 5.3 kilobases, which is much smaller than predicted from the size of paranemin (280 kDa) by sodium dodecyl sulf ate-polyacrylamide gel electrophoresis. The derived amino acid sequenc e of paranemin (1,606 residues; 178,161 kDa) contains the conserved IF rod domain (308 amino acids), which has highest homology to the rod d omains of nestin and tanabin. Thus, paranemin is an IF protein rather than an IFAP. Sequence analysis also revealed that the partial cDNA se quences of two proteins, namely EAP-300 and IFAPa-400, are almost iden tical to regions of the cDNA sequence of paranemin. The complete paran emin cDNA was expressed in a cell line (SW13) with, and without, detec table cytoplasmic IFs. Antibody labeling of these cells suggests that paranemin does not form IFs by itself, but rather is incorporated into heteropolymeric IFs with vimentin.