EXTRACELLULAR GLYCEROPHOSPHOLIPID-CHOLESTEROL ACYLTRANSFERASE FROM AEROMONAS-SALMONICIDA - ACTIVATION BY SERINE-PROTEASE

Extracellular haemolytic activities of Aeromonas salmonicida ssp. salmonicida to salmon red blood cells were shown to be due to different forms of the membrane-active enzyme glyccrophospholipid:cholesterol acyltransferase (GCAT). About 10% of the total haemolytic activity was due to a high molecular mass complex of LPS and GCAT (mol. mass >1000 kDa), containing 35-50%, neutral sugars and 1.5-2.0% protein. Some haemolytic activity (30-40%, of total), corresponding to 50-70 kDa by gel filtration, also contained GCAT-activity and map represent aggregated forms of GCAT. However, about 50% or more of the haemolytic activity was due to a protein of 26 kDa free GCAT. Rabbit antibodies to GCAT neutralized the haemolytic activity of both GCAT and GCAT-LPS. A transposon-produced serine protease negative mutant of the same A. salmonicida strain showed reduced haemolytic activity. The mutant produced a 38-kDa GCAT preform of low haemolytic activity. The preform was processed by autogenous serine protease to a highly haemolytic 26-kDa molecule with pI 63, similar to GCAT of the parent strain. The weakly haemolytic GCAT-LPS analogue of the mutant strain did not contain detectable amounts of the 26-kDa molecule and was not activated by proteases.