Function of cell-adhesion protein MFBA in fruiting-body formation of basidiomycete

担子菌出子是真核微生物中最典型的形态发生现象之一。目前已分离到与ras基因相邻且在成熟子实体中表达强烈的新基因mfbA。mfbA也在未成熟子实体中表达。克隆的mfbA cDNA全长6700 bp，编码2157个氨基酸。MFBA蛋白（MW234,494）含有纤维连接蛋白中首次发现的促进细胞表面附着的氨基酸序列Arg-Gly-Asp （RGD）。本研究旨在评估MFBA的功能。虽然我们没有得到完整的MFBA蛋白，但我们成功地在大肠杆菌中产生了含有RGD基序的425个氨基酸的MFBA片段（命名为MFBA（582-1006）肽）。利用该MFBA片段，得到了以下新的实验结果：(1) MFBA（582-1006）肽通过RGD序列对哺乳动物细胞表现出细胞粘附和扩散活性。(2)该肽通过RGD序列使酵母酵母酵母细胞在体内显著聚集。(3)该肽通过RGD序列引起担子菌菌丝细胞的体外聚集。(4) mfbA转录本在鳃缺失的菌毛和柄中丰富，但在鳃组织中几乎没有转录本。(5) MFBA（582-1006）肽在RGD基序附近具有高度疏水区域。这些结果强烈提示MFBA的疏水区嵌入担子菌菌丝细胞壁，含有RGD的氨基酸序列通过RGD序列与存在于其他细胞表面的受体蛋白分子相互作用。这些蛋白质之间的相互作用导致担子菌菌丝细胞的网状结构的形成。MFBA被认为参与了子实体的成熟和子实体的菌柄和柱头的形成。

Fruiting of basidiomycetes is one of the most typical morphogenetic phenomena observed in eukaryotic microorganisms. We so far isolated new gene mfbA which is adjacent to ras gene and expressed strongly in mature fruiting bodies. mfbA is also expressed in immature fruiting bodies. The mfbA cDNA cloned consisted of 6700 bp encoding 2157 amino acids. MFBA protein (MW234,494) contained the cell surface attachment-promoting amino acid sequence Arg-Gly-Asp (RGD) observed first in fibronectin.This research aimed to assess the function of MFBA.Although we failed in producing intact MFBA protein, a 425-amino-acid MFBA fragment containing the RGD motif (named MFBA (582-1006) peptide) was successfully producted in E.coli. By using this MFBA fragment, the following new experimental results were obtained.(1) MFBA (582-1006) peptide exhibits a cell adhesion and spreading activity toward mammalian cells via the RGD sequence.(2) The peptide causes significant in vivo aggrefation of the yeast S.cerevisiae cells via the RGD sequence.(3) The peptide causes significant in vitro aggregation of basidiomycetous hyphal cells via RGD sequence.(4) The mfbA transcript is abundant in the gill-depleted pileus and stipe, but almost no transcript is found in the gill tissue.(5) MFBA (582-1006) peptide possesses a highly hydrophobic region adjacent to the RGD motif.These results strongly suggest that the hydrophobic region of MFBA is embedded in basidiomycetous hyphal cell wall and the RGD-containing amino acid sequence interacts with the receptor protein molecules being present on the surface of other cells via the RGD sequence. These protein-protein interactions results in formation of the net structure of basidiomycetous hyphal cells. MFBA is considered to be involved in maturation of fruiting body and formation of pileus and stipe of fruiting body.