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J. Biol. Chem., Vol. 282, Issue 10, 7299-7311, March 9, 2007

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Maize -Glucosidase-aggregating Factor Is a Polyspecific Jacalin-related Chimeric Lectin, and Its Lectin Domain Is Responsible for -Glucosidase Aggregation^*

Farooqahmed S. Kittur, Mallikarjun Lalgondar, Hyun Young Yu, David R. Bevan, and Asim Esen¹

From the Department of Biological Sciences, and the Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0406

In certain maize genotypes, called "null," -glucosidase does not enter gels and therefore cannot be detected on zymograms after electrophoresis. Such genotypes were originally thought to be homozygous for a null allele at the glu1 gene and thus devoid of enzyme. We have shown that a -glucosidase-aggregating factor (BGAF) is responsible for the "null" phenotype. BGAF is a chimeric protein consisting of two distinct domains: the disease response or "dirigent" domain and the jacalin-related lectin (JRL) domain. First, it was not known whether the lectin domain in BGAF is functional. Second, it was not known which of the two BGAF domains is involved in -glucosidase binding and aggregation. To this end, we purified BGAF to homogeneity from a maize null inbred line called H95. The purified protein gave a single band on SDS-PAGE, and the native protein was a homodimer of 32-kDa monomers. Native and recombinant BGAF (produced in Escherichia coli) agglutinated rabbit erythrocytes, and various carbohydrates and glycoproteins inhibited their hemagglutination activity. Sugars did not have any effect on the binding of BGAF to the -glucosidase isozyme 1 (Glu1), and the BGAF-Glu1 complex could still bind lactosyl-agarose, indicating that the sugar-binding site of BGAF is distinct from the -glucosidase-binding site. Neither the dirigent nor the JRL domains alone (produced separately in E. coli) produced aggregates of Glu1 based on results from pull-down assays. However, gel shift and competitive binding assays indicated that the JRL domain binds -glucosidase without causing it to aggregate. These results with those from deletion mutagenesis and replacement of the JRL domain of a BGAF homolog from sorghum, which does not bind Glu1, with that from maize allowed us to conclude that the JRL domain of BGAF is responsible for its lectin and -glucosidase binding and aggregating activities.

Received for publication, August 4, 2006 , and in revised form, January 6, 2007.

^* This work was supported by National Science Foundation Grant MCB-0417143 (to A. E. and D. R. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0406. Tel.: 540-231-5894; Fax: 540-231-9307; E-mail: aevatan{at}vt.edu.

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