ABSTRACT
The origin of the high α(1,2-cis)-stereoselectivity in the reaction of galactosyl and galactosaminyl donors with a di-tert-butylsilylene (DTBS) group with several nucleophiles has been elucidated by means of experimental and computational approaches. DTBS overcomes any other cyclic protecting groups examined to date and the ß(1,2-trans)-directing effect due to the neighboring participation by CO groups at C2. Requirements for the α(1,2-cis)-stereoselectivity are as follows: (1) generation of an oxocarbenium ion; (2) a galacto-type glycosyl donor with a cyclic protecting group bridging O4 and O6 to form a six-membered ring; (3) through-space electron donation from O4 and O6 into the empty p-orbital of the anomeric carbon to stabilize the oxocarbenium intermediate; (4) steric hindrance due to bulky alkyl substituents on the cyclic protecting group to prevent nucleophilic attack from the ß-face; and (5) a 4,6-O-silylene structure. Furthermore, it was found that the strong stereodirecting effect of the DTBS group was unique and specific among the various cyclic protecting groups examined.
ABSTRACT
Tight junctions (TJs) and adherens junctions (AJs) are major junctional apparatuses in epithelial cells. Claudins and junctional adhesion molecules (JAMs) are major cell adhesion molecules (CAMs) at TJs, whereas cadherins and nectins are major CAMs at AJs. Claudins and JAMs are associated with ZO proteins, whereas cadherins are associated with beta- and alpha-catenins, and nectins are associated with afadin. We previously showed that nectins first form cell-cell adhesions where the cadherin-catenin complex is recruited to form AJs, followed by the recruitment of the JAM-ZO and claudin-ZO complexes to the apical side of AJs to form TJs. It is not fully understood how TJ components are recruited to the apical side of AJs. We studied the roles of afadin and ZO-1 in the formation of TJs in Madin-Darby canine kidney (MDCK) cells. Before the formation of TJs, ZO-1 interacted with afadin through the two proline-rich regions of afadin and the SH3 domain of ZO-1. During and after the formation of TJs, ZO-1 dissociated from afadin and associated with JAM-A. Knockdown of afadin impaired the formation of both AJs and TJs in MDCK cells, whereas knockdown of ZO-1 impaired the formation of TJs, but not AJs. Re-expression of full-length afadin restored the formation of both AJs and TJs in afadin-knockdown MDCK cells, whereas re-expression of afadin-DeltaPR1-2, which is incapable of binding to ZO-1, restored the formation of AJs, but not TJs. These results indicate that the transient interaction of afadin with ZO-1 is necessary for the formation of TJs in MDCK cells.
Subject(s)
Membrane Proteins/metabolism , Microfilament Proteins/metabolism , Phosphoproteins/metabolism , Tight Junctions/metabolism , Animals , Calcium/metabolism , Cell Line , Dogs , Membrane Proteins/genetics , Microfilament Proteins/genetics , Microscopy, Fluorescence , Phosphoproteins/genetics , Protein Binding/genetics , Protein Binding/physiology , Tight Junctions/genetics , Zonula Occludens-1 ProteinABSTRACT
Afadin is an actin-filament-binding protein that binds to nectin, an immunoglobulin-like cell-cell adhesion molecule, and plays an important role in the formation of adherens junctions. Here, we show that afadin, which did not bind to nectin and was localized at the leading edge of moving cells, has another role: enhancement of the directional, but not random, cell movement. When NIH3T3 cells were stimulated with platelet-derived growth factor (PDGF), afadin colocalized with PDGF receptor, alphavbeta3 integrin and nectin-like molecule-5 at the leading edge and facilitated the formation of leading-edge structures and directional cell movement in the direction of PDGF stimulation. However, these phenotypes were markedly perturbed by knockdown of afadin, and were dependent on the binding of afadin to active Rap1. Binding of Rap1 to afadin was necessary for the recruitment of afadin and the tyrosine phosphatase SHP-2 to the leading edge. SHP-2 was previously reported to tightly regulate the activation of PDGF receptor and its downstream signaling pathway for the formation of the leading edge. These results indicate that afadin has a novel role in PDGF-induced directional cell movement, presumably in cooperation with active Rap1 and SHP-2.
