Abstract
Profilins are low-molecular-mass (12-15 kDa) cytosolic proteins that are major regulators of actin assembly in all eukaryotic cells. In general, profilins from evolutionarily diverse organisms share the ability to bind to G-actin, poly-(L-proline) (PLP) and proline-rich proteins, and polyphosphoinositides. However, the functional importance of each of these interactions remains unclear and might differ between organisms. We investigated the importance of profilin's interaction with its various ligands in plant cells by characterizing four maize (Zea mays) profilin 5 (ZmPRO5) mutants that had single amino acid substitutions in the presumed sites of ligand interaction. Comparisons in vitro with wild-type ZmPRO5 showed that these mutations altered ligand association specifically. ZmPRO5-Y6F had a 3-fold increased affinity for PLP, ZmPRO5-Y6Q had a 5-fold decreased affinity for PLP, ZmPRO5-D8A had a 2-fold increased affinity for PtdIns(4,5)P(2) and ZmPRO5-K86A had a 35-fold decreased affinity for G-actin. When the profilins were microinjected into Tradescantia stamen hair cells, ZmPRO5-Y6F increased the rate of nuclear displacement in stamen hairs, whereas ZmPRO5-K86A decreased the rate. Mutants with a decreased affinity for PLP (ZmPRO5-Y6Q) or an enhanced affinity for PtdIns(4,5)P(2) (ZmPRO5-D8A) were not significantly different from wild-type ZmPRO5 in affecting nuclear position. These results indicate that plant profilin's association with G-actin is extremely important and further substantiate the simple model that profilin acts primarily as a G-actin-sequestering protein in plant cells. Furthermore, interaction with proline-rich binding partners might also contribute to regulating profilin's effect on actin assembly in plant cells.