Testicular protein kinase 1 (TESK1) is a serine/threonine kinase with a structure composed of a kinase domain related to those of LIM-kinases and a unique C-terminal proline-rich domain. Like LIM-kinases, TESK1 phosphorylated cofilin specifically at Ser-3, both in vitro and in vivo. When expressed in HeLa cells, TESK1 stimulated the formation of actin stress fibers and focal adhesions. In contrast to LIM-kinases, the kinase activity of TESK1 was not enhanced by Rho-associated kinase (ROCK) or p21-activated kinase, indicating that TESK1 is not their downstream effector. Both the kinase activity of TESK1 and the level of cofilin phosphorylation increased by plating cells on fibronectin. Y-27632, a specific inhibitor of ROCK, inhibited LIM-kinase-induced cofilin phosphorylation but did not affect fibronectin-induced or TESK1-induced cofilin phosphorylation in HeLa cells. Expression of a kinase-negative TESK1 suppressed cofilin phosphorylation and formation of stress fibers and focal adhesions induced in cells plated on fibronectin. These results suggest that TESK1 functions downstream of integrins and plays a key role in integrin-mediated actin reorganization, presumably through phosphorylating and inactivating cofilin. We propose that TESK1 and LIM-kinases commonly phosphorylate cofilin but are regulated in different ways and play distinct roles in actin reorganization in living cells.

Much progress defining the order and timing of endocytic internalization events has come as a result of real-time, live-cell fluorescence microscopy. Although the availability of numerous endocytic mutants makes yeast an especially valuable organism for functional analysis of endocytic dynamics, a serious limitation has been the lack of a fluorescent cargo for receptor-mediated endocytosis. We have now synthesized biologically active fluorescent mating-pheromone derivatives and demonstrated that receptormediated endocytosis in budding yeast occurs via the clathrin-and actin-mediated endocytosis pathway. We found that endocytic proteins first assemble into patches on the plasma membrane, and then ␣-factor associates with the patches. Internalization occurs next, concomitant with actin assembly at patches. Additionally, endocytic vesicles move toward early endosomes on actin cables. Early endosomes also associate with actin cables, and they actively move toward endocytic sites to capture vesicles being released from the plasma membrane. Thus, endocytic vesicle formation and capture of the newly released vesicles by early endosomes occur in a highly concerted manner, mediated by the actin cytoskeleton.actin ͉ cytoskeleton ͉ endocytosis ͉ endosome

The lipid phosphatidylinositol-4,5-bisphosphate (PtdIns[4,5]P2) appears to play an important role in endocytosis. However, the timing of its formation and turnover, and its specific functions at different stages during endocytic internalization, have not been established. In this study, Sla2 ANTH-GFP and Sjl2-3GFP were expressed as functional fusion proteins at endogenous levels to quantitatively explore PtdIns(4,5)P2 dynamics during endocytosis in yeast. Our results indicate that PtdIns(4,5)P2 levels increase and decline in conjunction with coat and actin assembly and disassembly, respectively. Live-cell image analysis of endocytic protein dynamics in an sjl1Δ sjl2Δ mutant, which has elevated PtdIns(4,5)P2 levels, revealed that the endocytic machinery is still able to assemble and disassemble dynamically, albeit nonproductively. The defects in the dynamic behavior of the various endocytic proteins in this double mutant suggest that PtdIns(4,5)P2 turnover is required for multiple stages during endocytic vesicle formation. Furthermore, our results indicate that PtdIns(4,5)P2 turnover may act in coordination with the Ark1/Prk1 protein kinases in stimulating disassembly of the endocytic machinery.

In both yeast and mammals, endocytic internalization is accompanied by a transient burst of actin polymerization. The yeast protein kinases Prk1p and Ark1p, which are related to the mammalian proteins GAK and AAK1, are key regulators of this process. However, the molecular mechanism(s) by which they regulate actin assembly at endocytic sites have not yet been determined. The Eps15-like yeast protein Pan1p is a Prk1p substrate that is essential for endocytic internalization and for proper actin organization. Pan1p is an Arp2/3 activator and here we show that this activity is dependent on F-actin binding. Mutation of all 15 Prk1p-targeted threonines in Pan1p to alanines mimicked the ark1Delta prk1Delta phenotype, demonstrating that Pan1p is a key Prk1p target in vivo. Moreover, phosphorylation by Prk1p inhibited the ability of Pan1p to bind to F-actin and to activate the Arp2/3 complex, thereby identifying the endocytic phosphoregulation mechanism of Prk1p. We conclude that Prk1p phosphorylation of Pan1p shuts off Arp2/3-mediated actin polymerization on endocytic vesicles, allowing them to fuse with endosomes.

We previously identified testicular protein kinase 1 (TESK1), which phosphorylates cofilin and induces actin cytoskeletal reorganization. We now report identification and characterization of another member of a TESK family, testicular protein kinase 2 (TESK2), with 48% amino acid identity with TESK1. Like TESK1, TESK2 phosphorylated cofilin specifically at Ser-3 and induced formation of actin stress fibers and focal adhesions. Both TESK1 and TESK2 are highly expressed in the testis, but in contrast to TESK1, which is predominantly expressed in testicular germ cells, TESK2 is expressed predominantly in nongerminal Sertoli cells. Thus, TESK1 and TESK2 seem to play distinct roles in spermatogenesis. In HeLa cells, TESK1 was localized mainly in the cytoplasm, whereas TESK2 was localized mainly in the nucleus, which means that TESK1 and TESK2 likely have distinct cellular functions. Because the kinase-inactive mutant of TESK2 was localized in the cytoplasm, nuclear/cytoplasmic localization of TESK2 depends on its kinase activity. A TESK2 mutant lacking the C-terminal noncatalytic region had about a 10-fold higher kinase activity in vitro and, when expressed in HeLa cells, induced punctate actin aggregates in the cytoplasm and unusual condensation and fragmentation of nuclei, followed by apoptosis. Thus, we propose that the C-terminal region plays important roles in regulating the kinase activity and cellular functions of TESK2.The dynamics of polymerization/depolymerization of actin filaments and their remodeling are essential for cell movement, adhesion, and division (1). Cofilin and actin-depolymerizing factor (ADF) 1 play an essential role in the rapid turnover of actin filaments and actin-based cytoskeletal reorganization by stimulating depolymerization and severance of actin filaments (2-4). As the activity of cofilin/ADF is negatively regulated by phosphorylation at Ser-3 (5), enzymes phosphorylating cofilin/ ADF seem to play important roles in actin filament dynamics. We and other investigators provide evidence that LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) (6 -8) phosphorylate cofilin/ADF specifically at Ser-3 and induce actin cytoskeletal reorganization by phosphorylating and inactivating cofilin (9, 10). LIM kinases are activated in cultured cells by Rho family small GTPases Rac, Rho, and Cdc42 (9 -11), this activation mediated by downstream effectors p21-activated kinase (PAK) and Rho-associated kinase, by phosphorylation of Thr-508 of LIMK1 or Thr-505 of LIMK2 (12-16).Testicular protein kinase 1 (TESK1) is a serine/threonine kinase with a structure composed of an N-terminal protein kinase domain and a C-terminal proline-rich region (17). The kinase domain of TESK1 is closely related to those of LIM kinases (17). We recently obtained evidence that TESK1, like LIM kinases, has the potential to phosphorylate cofilin/ADF specifically at Ser-3 and induces the formation of actin stress fibers and focal adhesions by phosphorylating cofilin/ADF (18). In contrast to LIM kinases, the kinase activity of TESK1 is...