Isolation and chemical composition of protein bodies and matrix proteins in corn endosperm

Abstract

Microscopic examination of endosperm cells reveals the presence of starch granules and protein bodies held together by a protein matrix. To separate these subcellular structures, endosperm tissue, obtained 24 days after pollination, was gently homogenized in phosphate buffer. The particulate cell components were then separated by zone sedimentation on a sucrose density gradient. Sequential fractions were taken from different depths of the gradient tube, analysed for protein, and examined microscopically. Proteins were characterized by their amino acid composition and starch-gel electrophoretic patterns. Comparisons were made with proteins obtained from endosperm meal by extraction with different solvents. The slowest-sedimenting protein zone contained typical albumins and globulins soluble in centrifugation buffer. A 2nd zone contained a high mol. wt. protein soluble only in alkali. The amino acid profile of this glutelin protein differed significantly from glutelin extracted from meal with alkali. The 3rd zone contained particles microscopically identical to free protein bodies. The protein in these bodies was characterized as zein. The 4th and fastest-sedimenting zone was identified as protein bodies surrounded by a protein matrix. The matrix protein, which resembled glutelin protein in its solubility properties, differs significantly from the extracted glutelin in amino acid composition. The electrophoretic patterns of the reduced and alkylated proteins of zones 2 and 4 when in combination, resemble the electrophoretic patterns of reduced and alkylated glutelin obtained by direct extraction of endosperm meal.