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Journal of Biochemistry Advance Access published online on March 15, 2008
Journal of Biochemistry, doi:10.1093/jb/mvn037
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© 2008 The Japanese Biochemical Society

Chemical modulation of the chaperone function of human {alpha}A-crystallin

Ashis Biswas1,*, Shawn Lewis1, Benlian Wang2,3, Masaru Miyagi2,3,4, Puttur Santoshkumar5, Mahesha H. Gangadhariah1 and Ram H. Nagaraj1,3,4,¶

Departments of 1Ophthalmology & Visual Sciences, 2Center for Proteomics, 3Visual Sciences Research Center and 4Department of Pharmacology, Case Western Reserve University, Cleveland, OH 44106and 5Mason Eye Institute, University of Missouri, Columbia, MO 65212.

Correspondence: Prof. Ram H. Nagaraj, Ph.D. Department of Ophthalmology, Case Western Reserve University 2085 Adelbert Road, Pathology Building, Room 311, Cleveland, OH 44106. Tel: 216-368-2089 FAX. 216-368-0363. e-mail: ram.nagaraj{at}case.edu

Received December 22, 2007; Accepted March 3, 2008


  Abstract

SUMMARY

{alpha}A-crystallin is abundant in the lens of the eye and acts as a molecular chaperone by preventing aggregation of denaturing proteins. We previously found that chemical modification of the guanidino group of selected arginine residues by a metabolic {alpha}-dicarbonyl compound, methylglyoxal (MGO), makes human {alpha}A-crystallin a better chaperone. Here, we examined how the introduction of additional guanidino groups and modification by MGO influence the structure and chaperone function of {alpha}A-crystallin. {alpha}A-crystallin lysine residues were converted to homoarginine by guanidination with o-methylisourea (OMIU) and then modified with MGO. LC-ESI-mass spectrometry identified homoargpyrimidine and homohydroimidazolone adducts after OMIU and MGO treatment. Treatment with 0.25 M OMIU abolished most of the chaperone function. However, subsequent treatment with 1.0 mM MGO not only restored the chaperone function but increased it by ~ 40% and ~ 60% beyond that of unmodified {alpha}A-crystallin, as measured with citrate synthase and insulin aggregation assays, respectively. OMIU reduced the surface hydrophobicity but after MGO treatment, it was ~39% higher than control. FRET analysis revealed that {alpha}A-crystallin subunit exchange rate was markedly retarded by OMIU modification, but was enhanced after MGO modification. These results indicate a pattern of loss and gain of chaperone function within the same protein that is associated with introduction of guanidino groups and their neutralization. These findings support our hypothesis that positively charged guanidino group on arginine residues keeps the chaperone function of {alpha}A-crystallin in check and that a metabolic {alpha}-dicarbonyl compound neutralizes this charge to restore and enhance chaperone function.

Key Words: {alpha}A-Crystallin, Chaperone, homoarginine, homoargpyrimidine, homohydroimidazolone

*Present address: Department of Pathobiology, Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, OH 44195, U.S.A.


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