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J Biol Chem, Vol. 275, Issue 3, 1897-1901, January 21, 2000

Primary Structure and Function Analysis of the Abrus precatorius Agglutinin A Chain by Site-directed Mutagenesis
Pro¹⁹⁹ OF AMPHIPHILIC -HELIX H IMPAIRS PROTEIN SYNTHESIS INHIBITORY ACTIVITY^*

Chao-Lin Liu, Chia-Chu Tsai, Su-Chang Lin^§, Li-In Wang, Chong-Ing Hsu, Ming-Jing Hwang^¶, and Jung-Yaw Lin

From the  Institute of Biochemistry, College of Medicine, National Taiwan University, the ^§ Institute of Life Sciences, National Defense Medical Center, and the ^¶ Institute of Biomedicine, Academia Sinica, Nankang, Taipei , Taiwan, Republic of China

Abrus agglutinin (AAG), a low-toxicity protein from the plant Abrus precatorius, is less lethal than abrina (ABRa) in mice (LD₅₀ = 5 mg/kg versus 20 µg/kg of body weight). Nucleotide sequence analysis of a cDNA clone encoding full-length AAG showed an open reading frame with 1641 base pairs, corresponding to a 547-amino acid residue preproprotein containing a signal peptide and a linker region (two amino acid residues) between the AAG-A and AAG-B subunits. AAG had high homology to ABRa (77.8%). The 13 amino acid residues involved in catalytic function, which are highly conserved among abrins and ricins, were also conserved within AAG-A. The protein synthesis inhibitory activity of AAG-A (IC₅₀ = 3.5 nM) was weaker than that of ABRa-A (0.05 nM). Molecular modeling followed by site-directed mutagenesis showed that Pro¹⁹⁹ of AAG-A, located in amphiphilic helix H and corresponding to Asn²⁰⁰ of ABRa-A, can induce bending of helix H. This bending would presumably affect the binding of AAG-A to its target sequence, GpApGpAp, in the tetraloop structure of the 28 S rRNA subunit and could be one of the major factors contributing to the relatively weak protein synthesis inhibitory activity and toxicity of AAG.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) AF190173.

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