PLANT AND ANIMAL PROFILINS ARE FUNCTIONALLY EQUIVALENT AND STABILIZE MICROFILAMENTS IN LIVING ANIMAL-CELLS

We have analyzed the degree of functional similarity between birch and mammalian profilins, two members of the profilin family which show on ly a moderate sequence homology (22%) in living animal cells, The plan t profilin, derived from birch pollen, was stably expressed in BHK-21 cells. Plant and endogenous profilin synthesis and cellular distributi on were monitored by specific monoclonal antibodies, Quantitation of p rofilin and actin on calibrated immunoblots showed that two stable clo nes contained in total 1.4 and 2.0 times as much profilin as the paren tal cells, Using double fluorescence and confocal laser scanning micro scopy, it was seen that the endogenous and the plant profilin colocali zed with dynamic microfilaments, in particular with F-actin-rich foci and cortical webs of spreading cells, with dynamic bundles induced by serum deprival, and with cytochalasin D- and latrunculin-induced trans ient F-actin aggregates. The increase in the overall profilin concentr ation correlated with a significantly higher resistance of actin filam ents to these drugs, Our data indicate that even profilins of highly d istant evolutionary origin can functionally substitute for each other and support the hypothesis that in animal cells, profilins are engaged in regulating either the stability or the kinetic properties of actin filaments.