Profilin Binding to Poly-l-Proline and Actin Monomers along with Ability to Catalyze Actin Nucleotide Exchange Is Required for Viability of Fission Yeast

Abstract

We tested the ability of 87 profilin point mutations to complement temperature-sensitive and null mutations of the single profilin gene of the fission yeast Schizosaccharomyces pombe. We compared the biochemical properties of 13 stable noncomplementing profilins with an equal number of complementing profilin mutants. A large quantitative database revealed the following: 1) in a profilin null background fission yeast grow normally with profilin mutations having Ͼ10% of wild-type affinity for actin or poly-l-proline, but lower affinity for either ligand is incompatible with life; 2) in the cdc3-124 profilin ts background, fission yeast function with profilin having only 2-5% wild-type affinity for actin or poly-l-proline; and 3) special mutations show that the ability of profilin to catalyze nucleotide exchange by actin is an essential function. Thus, poly-l-proline binding, actin binding, and actin nucleotide exchange are each independent requirements for profilin function in fission yeast. INTRODUCTIONProfilins are small proteins that interact with at least three types of ligands: actin monomers (and the homologous Arp2 subunit of Arp2/3 complex); proteins with sequences adopting a type II poly-l-proline helix; and polyphosphoinositides (Machesky and Pollard, 1993;Schluter et al., 1997). Interaction with actin monomers suppresses growth at the pointed end of actin filaments but not the barbed end (Pollard and Cooper, 1984). This allows profilin-actin complexes to support the growth of uncapped barbed ends in cells. In addition, most profilins (amoeba [Mockrin and Korn, 1980;Nishida, 1985], vertebrate [Goldschmidt-Clermont et al., 1991;Perelroizen et al., 1996], yeast [Eads et al., 1998]) but not all profilins (plants [Perelroizen et al., 1996;Eads et al., 1998;Kovar et al., 2000]) stimulate the exchange of the adenine nucleotide bound to actin monomers. This nucleotide exchange activity is postulated to prevent ADP-actin monomers dissociated from filaments from being trapped in highaffinity complexes with ADF/cofilins (Blanchoin and Pollard, 1998;Didry et al., 1998). Potential protein ligands with proline-rich sequences include VASP (Reinhard et al., 1995), Enabled (Ahern-Djamali et al., 1999), Mena (Gertler et al., 1996;Lanier et al., 1999), N-WASP , WAVE/Scar , verprolin/WIP (Ramesh et al., 1997), diaphanous/p140mDia (Watanabe et al., 1997), cappuccino (Manseau et al., 1996), Bni1p and Bnr1p (Evangelista et al., 1997;Imamura et al., 1997), cdc12 (Chang et al., 1997), drebrin and gephyrin (Mammoto et al., 1998), SMN (Giesemann et al., 1999), and aczonin (Wang et al., 1999). In the case of Mena, diaphanous, cappuccino, Bni1p, Bnr1p, and cdc12, genetic interactions support the biological significance of the interaction.Null or conditional mutations established that loss of profilin function is lethal in mice (Lanier et al., 1999), flies (Cooley et al., 1992;Manseau et al., 1996), and fission yeast (Balasubramanian et al., 1994) and severely disabling in Dictyostelium (Haugwitz et al., 1994) and budding yeast (Ha...

Profilin1 regulates PI(3,4)P 2 and lamellipodin accumulation at the leading edge thus influencing motility of MDA-MB-231 cells

“…Particularly, there is the issue of extent of changed affinity in vitro translating into cellular behaviors. For instance, a previous work on yeast showed that profilin1 mutants must have dramatically altered interactions to elicit changes in cellular responses (26). We have tried to account for this issue.…”

Structure and functions of profilins

“…These authors also reported a free energy change for unfolding of 5.9 and 1.8 kcal/ mol. Urea-induced unfolding studies were performed on various mutations of S. pombe profilin (Lu and Pollard 2001). The mid-point urea denaturation concentrations for most of the profilin mutants varied from 3.3 to 4.5 M. However, the L121R mutant showed a mid-point of denaturation at much lower urea concentration (1.6 M).…”

On the origin and evolution of vertebrate and viral profilins

“…Such studies have pointed to the fact that sequence similarity between profilins from invertebrate and vertebrate species is low, yet there is a remarkable conservation of 3D-structure and function [13,14]. For instance, the biochemically and structurally well-characterized profilins I and II from Acanthamoeba and Mammalia adopt the same fold [13] and bind actin, polyphosphoinositides and poly-L L-proline sequences [15], albeit that, in given species, for some isoforms differences in affinities for these ligands exist [8,16,17]. Aligning the primary structures of Acanthamoeba and Mammalian profilins, however, yields a similarity score lower than 25% and is thus in the twilight zone of evolutionary relationship based on sequence comparison.…”