Proteolysis of microtubule-associated protein 2 and tubulin by cathepsin D.
The in vitro degradation of microtubule-associated protein 2 (MAP-2) and tubulin by the lysosomal aspartyl endopeptidase cathepsin D was studied. MAP-2 was very sensitive to cathepsin D-induced hydrolysis in a relatively broad, acidic pH range (3.0-5.0). However, at a pH value of 5.5, cathepsin D-mediated hydrolysis of MAP-2 was significantly reduced and at pH 6.0 only a small amount of MAP-2 was degraded at 60 min. Interestingly, the two electrophoretic forms of MAP-2 showed different sensitivities to cathepsin D-induced degradation, with MAP-2b being significantly more resistant to hydrolysis than MAP-2a. To our knowledge, this is the first clear demonstration that MAP-2 is a substrate in vitro for cathepsin D. In contrast to MAP-2, tubulin was relatively resistant to cathepsin D-induced hydrolysis. At pH 3.5 and an enzyme-to-substrate ratio of 1: 20, only 35% of the tubulin was degraded by cathepsin D at 60 min. The cathepsin D-mediated hydrolysis of tubulin was optimal only at pH 4.5. These results demonstrate that MAP-2 and tubulin are unequally susceptible to degradation by cathepsin D. These data also imply a potential for rapid degradation of MAP-2 in vivo by cathepsin D either in lysosomes or perhaps autophagic vacuoles of the neuron.
Department of Neurology, University of Alabama, Birmingham 35294.
G V
GV
Johnson
J M
JM
Litersky
J N
JN
Whitaker
D000818:Animals; D001921:Brain / Q000201:enzymology; D002402:Cathepsin D / Q000302:isolation & purification / Q000378:metabolism*; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D010446:Peptide Fragments / Q000302:isolation & purification; D014404:Tubulin / Q000378:metabolism*
D008869:Microtubule-Associated Proteins; D010446:Peptide Fragments; D014404:Tubulin; D002402:Cathepsin D