DISTINCT BIOCHEMICAL CHARACTERISTICS OF THE 2 HUMAN PROFILIN ISOFORMS

The biochemical characteristics of a new human profilin isoform are de scribed. We refer to this recently described isoform as profilin II (i soelectric point 5.9) in comparison to profilin I (pI 8.4). We express ed both isoforms in bacteria and compared their actin-binding properti es, binding to poly(L-proline), affinities for phosphatidylinositol 4, 5-bisphosphate [PtdIns(4,5)P-2], and their effects on nucleotide excha nge on actin. Profilin I and profilin II have similar affinities for P tdIns(4,5)P-2 and poly(L-proline), and both accelerate nucleotide exch ange on monomeric actin to the same extent. However, the affinity of p rofilin I for monomeric actin is about five times higher than the affi nity of profilin II for actin. Potential structural differences of pro filin I and profilin II that might explain the difference in actin bin ding are discussed.