Alternative splicing of the mouse profilin II gene generates functionally different profilin isoforms

Profilins are a conserved family of proteins participating in actin dynamic s and cell motility, In the mouse, two profilin genes are known. Profilin I is expressed universally at high levels, while profilin II is expressed ma inly in the brain. Here we describe the occurrence of two mouse profilin II isoforms, A and B, which are derived by alternative splicing. They are ide ntical through residue 107 of the protein, but then have distinct C-termina l sequences. Profilin IIA binds to poly-L-proline and actin with high affin ity similar to profilin I. Profilin IIB on the other hand does not bind to actin and the affinity for poly-L-proline is greatly diminished. However, t ubulin was found to bind to GST-profilin IIB, and in vivo GFP-profilin IIB was recruited to spindles and asters during mitosis in HeLa cells. Our resu lts indicate unexpected diversity in the functions of the profilin family o f proteins, and suggest that in mouse profilin IIA is intimately involved i n actin dynamics, while profilin IIB associates with other cytoskeletal com ponents.