POLLEN PROFILIN FUNCTION DEPENDS ON INTERACTION WITH PROLINE-RICH MOTIFS

The actin binding protein profilin has dramatic effects on actin polym erization in vitro and in living cells. Plants have large multigene fa milies encoding profilins, and many cells or tissues can express multi ple profilin isoforms, Recently, we characterized several profilin iso forms from maize pollen for their ability to alter cytoarchitecture wh en microinjected into living plant cells and for their association wit h poly-L-proline and monomeric actin from maize pollen. In this study, we characterize a new profilin isoform from maize, which has been des ignated ZmPRO4, that is expressed predominantly in endosperm but is al so found at low levels in all tissues examined, including mature and g erminated pollen. The affinity of ZmPRO4 for monomeric actin, which wa s measured by two independent methods, is similar to that of the three profilin isoforms previously identified in pollen. In contrast, the a ffinity of ZmPRO4 for poly-L-proline is nearly twofold higher than tha t of native pollen profilin and the other recombinant profilin isoform s. When ZmPRO4 was microinjected into plant cells, the effect on actin -dependent nuclear position was significantly more rapid than that of another pollen profilin isoform, ZmPRO1. A gain-of-function mutant (Zm PRO1-Y6F) was created and found to enhance poly-L-proline binding acti vity and to disrupt cytoarchitecture as effectively as ZmPRO4. In this study, we demonstrate that profilin isoforms expressed in a single ce ll can have different effects on actin in living cells and that the po ly-L-proline binding function of profilin may have important consequen ces for the regulation of actin cytoskeletal dynamics in plant cells.