Involved in the intracellular degradation of the cyclic tetrasaccharide cyclobis-(1-6)-alpha-nigerosyl (CNN) formed extracellularly from starch. Catalyzes the hydrolysis of the alpha-1,3-glucosidic linkage of cyclobis-(1-6)-alpha-nigerosyl (CNN) to yield isomaltose via a possible linear tetrasaccharide. It has a strong preference for the alpha-(1-3)-isomaltosyl moiety. cyclobis-(1->3)-alpha-D-isomaltosyl + 2 H2O = 2 isomaltose kcat is 22.3 sec(-1) for cyclobis-(1-6)-alpha-nigerosyl (CNN) as substrate. Optimum pH is 7.9. Optimum temperature is under 41 degrees Celsius. Belongs to the glycosyl hydrolase 31 family.