Species differences in the sites of cleavage of pro-lactase to lactase supports lack of selective pressure.
The pro-sequences in pro-lactase-phlorizin hydrolase (LPH) are needed for lactase to proceed past the ER, but are irrelevant as to the enzymatic activities. Hence, in all species removal of the pro- sequences (or most of them) must take place after the ER. Contrary to this, the details of the removal of these pro-sequences are to be expected to differ in the various species, since they are not subjected to selective pressure. Using site-directed mutagenesis we investigated processing in rabbit. The first cleavage occurs by furin (or furin-like PCs) and takes place at R-A-A-R(349) in the pro-sequence, generating the known 180 kDa intermediate. Replacing R(349) by Q results in a mutant which is not cleaved but nevertheless transported to the cell surface as demonstrated by immunofluorescence. Further processing of either the 180 kDa intermediate or the mutant is not directly mediated by furin-like PCs, but involves (also) other proteases. These results demonstrate that formation of the 180 kDa intermediate, consistently found only in rabbits, but not in man, is not essential for lactase transport: in all likelihood lack of selective pressure has led to species-specific processing of pro-LPH.
Zecca
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Institute of Biochemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092, Postfach 35, Zurich, Switzerland.
Mesonero
J E
JE
Gloor
S M
SM
Semenza
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G
D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D004792:Enzyme Precursors / Q000737:chemistry* / Q000235:genetics; D005455:Fluorescent Antibody Technique; D045683:Furin; D006801:Humans; D043322:Lactase; D010694:Lactase-Phlorizin Hydrolase / Q000737:chemistry* / Q000235:genetics; D008970:Molecular Weight; D016297:Mutagenesis, Site-Directed; D011312:Pressure; D011817:Rabbits; D013045:Species Specificity; D013381:Subtilisins; D014162:Transfection; D001616:beta-Galactosidase / Q000737:chemistry* / Q000235:genetics
D004792:Enzyme Precursors; C096385:pro-lactase-phlorizin hydrolase; D043322:Lactase; D001616:beta-Galactosidase; D010694:Lactase-Phlorizin Hydrolase; D013381:Subtilisins; D045683:Furin