Abstract
The present study established that there are two distinct polymeric forms of isoamylase1 (ISA1) in rice endosperm: presumably a homo-pentamer of ISA1 and a ISA1-2 hetero-hexamer composed of five ISA1 and one ISA2. The molecular size of homo- and hetero-multimers were approximately 420 to 480 kDa and 510 to 550 kDa, respectively. The ISA1-2 multimer exhibited higher affinities for various branched polyglucans, especially for phytolglycogen, which had a Km value that was approximately 12 times lower relative to that with the ISA1 complex. The ISA1 complex was active even when incubated at 50 degree for 10 min, while the homo-multimer was completely inactivated at 40 degree for 10 min. These results suggest that the ISA1-2 multimer plays a predominant role in the amylopectin biosynthesis in rice endosperm although the ISA1 multimer can complement the function of the ISA1-2 multimer at least to some extent.
