Two sequence‐unrelated families of proteins possess peptidylproline cis‐trans‐isomerase activities (PPlase). PPlases are highly sequence conserved and multifunctional proteins which are present in many types of cells with a considerably divergent phylogenetic distribution. On the cellular level, PPlases occur in every compartment, both as free species and anchored to membranes. Diverse posttranslational modifications such as glycosylation, N‐terminal modifications and phosphorylation constitute the additional functional features of PPlase. Folding, assembly and trafficking of proteins in the cellular milieu are regulated by PPlase. These enzymes accelerate the rate of in‐vitro protein folding and they have the ability to bind proteins and act as chaperones. Some PPlases are coregulatory subunits of molecular complexes including heat‐shock proteins, glucocortcoid receptors and ion channels. Secreted forms of PPlases are inflammatory and chemotactic agents for monocytes, eosinophils and basophils. The potent and clinically useful immunosuppressants CsA, FK506 or rapamycin bind with high affinities to PPlases (immunophilins). The binding criterion allows us to sort the PPlases for the
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