A calcium-stimulated serine peptidase from a true-branching cyanobacterium, Westiellopsis ramosa sp. nov.

Abstract

Unbranched heterocytous cyanobacteria produce a number of serine peptidases. We have characterized several peptidases in the cell-free extracts of a true-branched N₂-fixing cyanobacterium, Westiellopsis ramosa sp. nov. Upon substrate-gel zymography of intact filaments and heterocytes, five peptidase bands were resolved, whereas in vegetative cells, a single band was discernible. No band was detected in ${\text{NO}}_{3}^{ - } /{\text{NH}}_{4}^{ + }$-grown cultures suggesting that the peptidases were present under diazotrophic conditions with much of them confined to heterocytes. Using salt precipitation and chromatography, a caseinolytic peptidase, called Wrp49, was purified which also demonstrated fibrinolytic activity. In SDS-PAGE, the purified peptidase was resolved into 17 and 27 kDa fragments. The enzyme in its native state exhibited M_r ≈ 49 kDa, and digested gelatin in a substrate gel at a corresponding position. The enzyme showed amidolytic activity on a plasmin specific substrate, D-Val-Leu-Lys p-nitroanilide. Moreover, a trypsin specific substrate, N-benzoyl-DL-Arg p-nitroanilide was hydrolyzed at an apparent K_m = 0.195 mM and V_max = 5 × 10⁻⁷ M s⁻¹. The enzyme was stable in a wide pH and temperature range. While Ca²⁺ stimulated the activity; phenylmethane sulfonyl fluoride, leupeptin, EDTA and chelants were inhibitory. The activity of the EDTA-inactivated enzyme was completely restored upon adding Ca²⁺, suggesting that both compounds competed with each other in modulating the enzyme activity. The enzyme showed similarities with a Ca²⁺ stimulated subtilisin-like serine peptidase of Anabaena variabilis ATCC 29413, but also presented several unique features of metallopeptidases, such as the chelant’s response. Moreover, the N-terminal sequence (MTVENLARTGVGPGWR) did not match with any of the known peptidases.