Mitchell M A, Huang M M, Chien P, Indik Z K, Pan X Q, Schreiber A D
Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia.
Blood. 1994 Sep 15;84(6):1753-9.
Fc gamma RIIA in the absence of other Fc receptors or receptor subunits induces the ingestion of IgG-coated cells. The cytoplasmic domain of Fc gamma RIIA contains two Y-x-x-L sequences similar to those in other Ig gene family receptors plus an additional tyrosine residue not in a Y-x-x-L motif. Upon cross-linking, Fc gamma RIIA is phosphorylated on tyrosine and the cytoplasmic tyrosines, Y275 (Y1), Y282 (Y2), and Y298 (Y3), may be important for its phagocytic activity. Because COS-1 cells can serve as a model for examining molecular structures involved in phagocytosis, substitutions and deletions were introduced into the cytoplasmic domain of Fc gamma RIIA and examined in COS-1 cell transfectants for their effects on phagocytosis and tyrosine phosphorylation. Disruption of a single cytoplasmic Y-x-x-L motif by substitution of tyrosine Y2 or Y3 by phenylalanine or by removing the threonine and leucine residues within the motif inhibited phagocytosis 50% to 65%. Tyrosine phosphorylation of Fc gamma RIIA also was inhibited, although to a greater extent by the substitution of Y3 than of Y2. Replacement of the N-terminal first cytoplasmic domain tyrosine, Y1, which is not within a typical Y-x-x-L, by itself did not inhibit phagocytosis, but replacement of Y1 in mutants lacking Y2 or Y3 virtually eliminated phagocytic activity and receptor tyrosine phosphorylation. Thus, at least two cytoplasmic tyrosines, including at least one typical single Y-x-x-L motif, are required for phagocytosis by Fc gamma RIIA. The data suggest that there is a close but not a simple relationship between phosphorylation of the Fc gamma RIIA cytoplasmic tyrosines and Fc gamma RIIA-mediated phagocytosis. Y3 appears to be particularly important because its removal by truncation or replacement with phenylalanine inhibits both tyrosine phosphorylation and phagocytosis in parallel. Alterations in the 12 residue proline-containing sequence between the two Y-x-x-L motifs also reduced phagocytic activity and tyrosine phosphorylation. Thus, the specific structure of the Fc gamma RIIA cytoplasmic domain accounts for its ability to stimulate phagocytosis in the absence of other subunits.
在缺乏其他Fc受体或受体亚基的情况下，FcγRIIA可诱导摄取IgG包被的细胞。FcγRIIA的胞质结构域包含两个与其他Ig基因家族受体中相似的Y-x-x-L序列，外加一个不在Y-x-x-L基序中的额外酪氨酸残基。交联后，FcγRIIA的酪氨酸发生磷酸化，胞质酪氨酸Y275（Y1）、Y282（Y2）和Y298（Y3）可能对其吞噬活性很重要。由于COS-1细胞可作为研究吞噬作用相关分子结构的模型，因此将FcγRIIA胞质结构域进行了取代和缺失操作，并在COS-1细胞转染体中检测其对吞噬作用和酪氨酸磷酸化的影响。通过用苯丙氨酸取代酪氨酸Y2或Y3，或去除基序中的苏氨酸和亮氨酸残基来破坏单个胞质Y-x-x-L基序，可使吞噬作用抑制50%至65%。FcγRIIA的酪氨酸磷酸化也受到抑制，不过Y3取代比Y2取代的抑制程度更大。取代不在典型Y-x-x-L基序中的N端第一个胞质结构域酪氨酸Y1本身并不抑制吞噬作用，但在缺乏Y2或Y3的突变体中取代Y1实际上消除了吞噬活性和受体酪氨酸磷酸化。因此，FcγRIIA的吞噬作用至少需要两个胞质酪氨酸，包括至少一个典型的单个Y-x-x-L基序。数据表明，FcγRIIA胞质酪氨酸的磷酸化与FcγRIIA介导的吞噬作用之间存在密切但并非简单的关系。Y3似乎尤为重要，因为通过截短或用苯丙氨酸取代将其去除会同时抑制酪氨酸磷酸化和吞噬作用。两个Y-x-x-L基序之间含脯氨酸的12个残基序列的改变也降低了吞噬活性和酪氨酸磷酸化。因此，FcγRIIA胞质结构域的特定结构决定了其在缺乏其他亚基时刺激吞噬作用的能力。
