電子顕微鏡でみた小腸微絨毛膜タンパク質のトポグラフィー
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Recent ultrastructural studies have revealed certain topographical characteristics of intestinal microvillar membrane proteins. When viewed by electron microscopy of conventional thin sections prepared from isolated microvilli, the external surface of the membrane is very poorly overlaid with fuzzy structures. However, negative staining electron microscopy shows that the external surface is studded with numerous dimeric particles and sometimes with doughnut-shaped particles, which are known to be hydrolytic enzymes abundant in the microvillar membrane. The freeze-fracture faces of the microvillar membrane numerous intramembrane particles, which are presumed to represent integral membrane proteins, e.g. transporters. The large part of the present review is concerned with the hydrolytic enzymes, especially sucrase-isomaltase complex, the enzyme most studied ultrastructurally. The hydrolytic enzymes have certain common structural features. They consist of two similar or dissimlar subunits, though a few exceptions are known. The major hydrophilic part of the molecule contains the enzymic active site and protrudes from the external surface. The minor hydrophbic domain is responsible for anchoring the molecule to the membrane and located near the amino terminus of the polypeptide chain. It is likely that for the symmetric enzymes, e.g. pig aminopeptidases A and M both subunits are associated with the membrane, while for the asymmetric enzymes, e.g. sucrase-isomaltase complex and gamma-glutamylpeptidase only one subunit is bound to the membrane. The relationship between the microvillar membrane proteins and the core cytoskeleton remains to be elucidated
- 日本膜学会の論文
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