Actin binding and proline rich motifs of CR16 play redundant role in growth of vrp1Delta cells

Enteropathogenic Escherichia coli and vaccinia virus do not require the family of WASP-interacting proteins for pathogen-induced actin assembly

The human pathogens enteropathogenic Escherichia coli (EPEC) and vaccinia virus trigger actin assembly in host cells by activating the host adaptor Nck and the actin nucleation promoter neural Wiskott-Aldrich syndrome protein (N-WASP). EPEC translocates effector molecules into host cells via type III secretion, and the interaction between the translocated intimin receptor (Tir) and the bacterial membrane protein intimin stimulates Nck and N-WASP recruitment, leading to the formation of actin pedestals beneath adherent bacteria. Vaccinia virus also recruits Nck and N-WASP to generate actin tails that promote cell-to-cell spread of the virus. In addition to Nck and N-WASP, WASP-interacting protein (WIP) localizes to vaccinia virus tails, and inhibition of actin tail formation upon ectopic expression of WIP mutants led to the suggestion that WIP is required for this process. Similar studies of WIP mutants, however, did not affect the ability of EPEC to form actin pedestals, arguing against an essential role for WIP in EPEC-induced actin assembly. In this study, we demonstrate that Nck and N-WASP are normally recruited by vaccinia virus and EPEC in the absence of WIP, and neither WIP nor the WIP family members CR16 and WIRE/WICH are essential for pathogen induced actin assembly. In addition, although Nck binds EPEC Tir directly, N-WASP is required for its localization during pedestal formation. Overall, these data highlight similar pathogenic strategies shared by EPEC and vaccinia virus by demonstrating a requirement for both Nck and N-WASP, but not WIP or WIP family members in pathogen-induced actin assembly.

Vrp1p-Las17p interaction is critical for actin patch polarization but is not essential for growth or fluid phase endocytosis in S. cerevisiae

Vrp1p (yeast WIP) forms a protein complex with Las17p (yeast WASP), however the physiological significance of the interaction has not been fully characterized. Vrp1p residues, (788)MPKPR(792) are essential for Vrp1p-Las17p interaction. While C-Vrp1p(364-817) complements all the defects of the vrp1Δ strain, C-Vrp1p(364-817)(5A) ((788)AAAAA(792)) does not complement any of the defects, due to its inability to localize to cortical patches. Targeting C-Vrp1p(364-817)(5A) to membranes using CAAX motif (C-Vrp1p(364-817)(5A)-CAAX) rescued the growth and endocytosis defect but not the actin patch polarization defect of vrp1Δ. Vrp1p can localize to cortical patches, either by binding to Las17p through LBD (Las17 Binding Domain, Vrp1p(760-817)) or independent of Las17p through residues in N-Vrp1p(1-364). Unlike Vrp1p, Vrp1p(5A) localizes poorly to cortical patches and complements all the defects of vrp1Δ strain except actin patch polarization at elevated temperature. N-Vrp1p(1-364) complements all the defects of vrp1Δ strain except the actin patch polarization defect while N-Vrp1p(1-364)-LBD fusion protein complements all the defects. Thus our results show that while both Vrp1p and Las17p are essential for many cellular processes, the two proteins do not necessarily have to bind to each other to carry out these cellular functions. However, Las17p-Vrp1p interaction is essential for actin patch polarization at elevated temperature.

Analysis of conformational changes in WASP using a split YFP

WASP (Wiskott-Aldrich syndrome protein) has been proposed to adopt a closed conformation (autoinhibited conformation) due to interaction between the carboxy terminal and the GTPase binding domain. Various WASP-interacting proteins have been suggested to relieve this autoinhibition. We have used the split YFP (Yellow Fluorescent Protein) to analyze the conformation of WASP. Saccharomyces cerevisiae cells expressing YFP1-154-WASP-YFP155-238 were found to exhibit YFP fluorescence while cells expressing of YFP1-154-WASP and WASP-YFP155-238 did not suggesting an intramolecular complementation of the YFP molecule. The fluorescence signal of YFP1-154-WASP-YFP155-238 was enhanced in the presence of WIP (WASP-interacting protein) however this is not due to the increased stability of YFP1-154-WASP-YFP155-238. Expression of Toca-1 and Nck1 reduced the YFP fluorescence from YFP1-154-WASP-YFP155-238 even in the presence of WIP suggesting that binding of Toca-1 or Nck1 altered the conformation of YFP1-154-WASP-YFP155-238. Thus both Nck1 and Toca-1 can relieve the autoinhibition of the WASP molecule.

Structure-function analysis of the WIP role in T cell receptor-stimulated NFAT activation: evidence that WIP-WASP dissociation is not required and that the WIP NH2 terminus is inhibitory

WASP and its binding partner WIP play important roles in T cells both in actin polymerization and in interleukin-2 transcription. Aberrations thereof contribute to the pathology of Wiskott-Aldrich syndrome (WAS). To directly evaluate the cooperativity of WIP and WASP in interleukin-2 transcription, we investigated how the WIP-WASP complex regulates NF-AT-mediated gene transcription. We developed an improved model system for analysis, using WIP and WASP cotransfection into Jurkat cells, in which strong induction of NFAT reporter activation is observed with anti-T cell receptor (TCR) antibody without the phorbol 12-myristate 13-acetate usually used previously. Using this system, our findings contradict a prevailing conceptual model of TCR-induced WIP-WASP dissociation by showing in three ways that the WIP-WASP complex mediates TCR-induced NFAT activation without dissociation. First, phosphorylation of WIP Ser(488) does not cause dissociation of the WIP-WASP complex. Second, WIP-WASP complexes do not dissociate demonstrably after TCR stimulation. Third, a fusion protein of WIP to WASP efficiently mediates NFAT activation. Next, our studies clarify that WIP stabilization of WASP explains otherwise unexpected results in TCR-induced NFAT activation. Finally, we find that the NH(2) terminus of WIP is a highly inhibitory region for TCR-mediated transcriptional activation in which at least two elements contribute: the NH(2)-terminal polyproline and the NH(2)-terminal actin-binding WH2 domain. This suggests that WIP, like WASP, is subject to autoinhibition. Our data indicate that the WIP-WASP complex plays an important role in WASP stabilization and NFAT activation.