| Research Abstract |
Our recent studies on band 3 protein, a typical multi-spanning polytopic membrane protein, implied that some hydrophobicpeptideportions were found as free from boundary lipids even though their hydrophobicity was comparable to that of bound from (Hamasaki, et al., J. Biochem. 122, 577-585(1997)). Moreover, the anticipated transmembrane peptide portions of band 3 protein were not equivalent to each other and some of them had no topogenic signal activities (Ota, et al., Biol. Chem. 273, 28286-28291(1998)). It was also found that topogenic signal activities were affected by the mutual peptide-peptide interactions of the transmembrane peptide portions during biosynthesis in vitro (Ota, et al., Biol. Chem. 273, 28286-28291(1998)). Intriguingly, we also observed the evidence for a mode of co-translational insertion in which an internal signal-another sequence with Nexo/Ccyt topology conferred a transmembrane disposition onto a preceding hydrophilic peptide segment (Ota, et al., Molecular Cel
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l 2, 495-503(1998)), suggesting that hydrophilic peptide portions can transverse the membrane lipid bilayer surrounded by other trasnmembrane peptide portions (especially with amphipathic transmembrane peptide portions). It is highly probable to speculated that the mutual peptide-peptide interactions of multi-spanning polytopic membrane proteins occurred throughout the protein biosynthesis and plasma membrane intersection events (Hamasaki, et al., Biochem. Cell., Biol. 76, 729-733(1998)).
Based on this evidence, it can be concluded that the hydrophobic transmembrane peptide portions are not necessarily bound with boundary lipids in the membrane lipid bilayer. Thus, hydrophobicity is not an absolute requirement for the formation of a transmembrane segment in multispanning polytopic membrane proteins. Category 2 portions (see J. Biochem. 122, 577-585 (1997) ; Biochem. Cell Biol. 79, 729-733 (1998)) have considerable freedom in the membrane lipid bilayer. Proteins containing category 2 portions have a more flexible structure than membrane proteins that consist only of transmembrane peptide portions bound with boundary lipids. These flexible regions must play important roles in substrate-peptide interactions or ligand-mediated conformational change within membrane lipid bilayers. Further study on other polytopic membrane proteins would support this new concept. Less
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