Ultrastructural and biochemical properties of the 120-kDa form of chick kinectin

Kinectin, an integral membrane protein (160 kDa), was identified as a kinesin-binding protein. Analysis of the predicted amino acid sequence of kinectin cDNA indicated an α-helical coiled-coil structure from amino acid 320 to 1310. A 120-kDa kinectin has been observed consistently, and N- terminal sequencing showed that 232 amino acids were missing from the N terminus of full-length kinectin. 120-kDa kinectin was distributed in the supernatant and a low density fraction of vesicles, whereas both forms were in the high density fraction of vesicles. In the electron microscope, the 120-kDa form appeared as a linear molecule of 133 nm in length. In hydrodynamic studies, the cytosolic 120-kDa kinectin was a dimer. Monoclonal antibody molecules (anti-kinectin KR160.9) bound asymmetrically to kinectin often with two antibodies/kinectin, indicative of a parallel coiled-coil. Metabolic labeling with [³H]myristic acid showed that both the 120- and 160- kDa kinectin are myristoylated in chick embryo fibroblasts. The myristoylation of 120-kDa kinectin may provide a mechanism for linking it to a low density fraction of vesicles. Immunoprecipitation with a 160-kDa kinectin-specific antibody brought down the 120-kDa kinectin. Thus, we suggest that kinectin is an extended parallel coiled-coil dimer, often a heterodimer.