A serine-type protease activity of human lens βA3-crystallin is responsible for its autodegradation

The purpose of the study was to determine whether the autodegradation of human βA3-crystallin is due to its intrinsic protease activity.

Recombinant His-tagged human βA3-crystallin was expressed in E. coli and purified by a Ni+2-affinity column chromatographic method. To determine protease activity, the purified crystallin was incubated for 24 h with either sodium deoxycholate, Triton X-100, or CHAPS {3-[(3-Cholamidopropyl) dimethylammonio]-1-propanesulfonate} and with benzoyl DL-arginine p-nitroanilide (BAPNA), a colorimetric protease substrate. The autodegradation of the crystallin at 0 h and 24 h on incubation at 37 °C with and without detergents (CHAPS/Triton X-100) was also determined by sodium dodecylsulfate-PAGE (SDS-PAGE) method. To examine whether the autodegradation of the crystallin was due to its protease activity, the crystallin was incubated with inhibitors of serine-, metallo- and cysteine-proteases. The binding of the intact βA3-crystallin and its autodegradation products to FFCK [5-carboxyfluorescenyl-1-phenylalaninyl-chloromethyl ketone], an analog of TPCK [1-Chloro-3-tosylamido-4-phenyl-2-butanone, a chymotrypsin-type serine protease inhibitor] was also determined by their incubation followed by SDS-PAGE and scanning for fluorescence using a Typhoon 9400 scanner.

βA3-crystallin protease activity showed activation in the presence of CHAPS but not in presence of Triton X-100. Upon incubation of βA3-crystallin for 24 h with CHAPS or sodium deoxycholate and BAPNA as a substrate, a time-dependent increase in the Arg-bond hydrolyzing activity was observed. SDS-PAGE analysis exhibited autodegradation products with Mr of 22, 27 and 30 kDa, which on partial NH2-terminal sequencing showed cleavage of Lys17-Met18, Gln4-Ala5 and Thr-Gly (in the NH2-terminal His-tag region) bonds, respectively. Almost no autodegradation of the βA3-crystallin occurred during its incubation alone or with CHAPS plus serine protease inhibitors (phenylmethylsulfonyl fluoride [PMSF], approtinin, and chymostatin). In contrast, the autodegradation occurred in the presence of metallo-protease inhibitors (EDTA and EGTA) and cysteine protease inhibitors (E-64, N-methylmaleimide and iodoacetamide). The βA3-crystallin also exhibited binding to FFCK, suggesting existence of a chymotrypsin-type active site in the βA3-crystallin protease.

The results suggested that a serine-type protease activity of βA3-crystalllin was responsible for its autodegradation. The specific bonds cleaved during autodegradation (Gln4-Ala5 and Lys17-Met18), were localized in the NH2-terminal arm of βA3-crystallin.