α 2 -μ-Globulin (A 2 ), an 18.6kDa protein produced in the liver, accumulates in the proximal tubule as a 15.5kDa cleavage product called "α 2μ-globulin fragment" (A 2 -f). A 2 -f possesses several unusual properties: (i) it escapes lysosomal degradation in the proximal tubules which is the common degradative pathway for all other proteins processed at this site; (ii) it accumulates selectively in the cytosol of proximal tubule, where it may represent the most abundant protein; and (iii) it

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... ses a biological membrane to enter the cytosol. The structural features that mediate this unusual behavior have never been identified.