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

担子菌结实是真核微生物中最典型的形态发生现象之一。到目前为止，我们分离到了与ras基因相邻的在成熟子实体中强表达的新基因MFBA。MFBA也在未成熟的子实体中表达。克隆的MFBA基因全长6700bp，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)，本研究旨在研究MFBA的功能，虽然我们未能获得完整的MFBA蛋白，但我们在大肠杆菌中成功地表达了含有RGD基序的425个氨基酸的MFBA片段(命名为MFBA(582-1006)肽)。通过使用该MFBA片段，获得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具有细胞黏附和扩散活性。(2)MFBA(582-1006)肽通过RGD序列在体内引起酵母细胞的显著聚集。(3)MFBA(582-1006)肽通过RGD序列引起担子菌丝细胞的体外聚集。(4)MFBA转录本在去鳃的菌盖和菌柄中丰富，(5)MFBA(582-1006)多肽在担子菌菌丝细胞壁上有一个与RGD基序相邻的高度疏水区，表明MFBA的疏水区嵌入担子菌菌丝细胞壁，含RGD的氨基酸序列通过RGD序列与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网状结构的形成。MFBA被认为与子实体的成熟、子实体菌盖和菌柄的形成有关。

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.