A 33 kDa protein with sequence homology to the ‘laminin binding protein’ is associated with the cytoskeleton in hydra and in mammalian cells

In hydra and in mammalian cells the monoclonal antibody V recognises an epitope which colocalises with cytoskeletal structures. Using this antibody for expression screening, a cDNA clone (955 bp) was isolated from hydra, which covers an open reading frame for a protein of 294 amino acids with a calculated molecular mass of 32.8 kDa. Northern blot analysis of hydra RNA resulted in a single mRNA species of 1.2 kb, and primer extension experiments proved this to be the full length message. 218 residues at the amino terminus of the hydra protein show extensive homology (73.5%) to a human protein designated ‘laminin binding protein’. The carboxyl-terminal 76 amino acids possess no significant similarity (20%). The monoclonal antibody V, which recognises an epitope in this carboxyl-terminal part, reacts in Western blots, both in hydra and in mammalian cells, with a protein of 33 kDa and not with the 45 kDa ‘laminin binding protein’. The 33 kDa protein is not extracellular or transmembrane, but has a strictly intracellular location as indicated by its amino acid sequence and by immunocytochemical and cell fractionation studies. In non-dividing mammalian cells the 33 kDa protein colocalises with filamentous structures; in dividing cells it dissociates from it and concentrates centrally. Presence of the SPLR-sequence, which is the consensus phosphorylation motif for the p34cdc2 kinase, links this 33 kDa protein to events occurring during the cell cycle.