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S. P. Bhat, R. K. Gangalum; Is B-Crystallin a Soluble Protein?. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1528.
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Among crystallins, a small fraction of α-crystallins have been historically known to associate with membranous components in the cell. We have recently demonstrated a significant association of αB-crystallin with perinulcear Golgi in the human Glioblastoma U373 cell line. Additional studies have established that αB-crystallin is also associated with Golgi in the ocular lens. In order to understand the functional import of the association of αB-crystallin with the membrane (in the backdrop of the dogma that this protein belongs to the soluble compartment); present studies were conducted to investigate the status of this protein in various membrane fractions isolated from the developing ocular lens.
Rat lenses of various ages were used for this study. Discontinuous sucrose density gradients were used to enrich for Golgi membrane and high density vesicular membrane fractions. Proteins were identified by immunoblotting. Light and confocal microscopy were used to examine the status of αB-crystallin in the various cellular and metabolic compartments of the ocular lens.
In this investigation we show that in the fetal and early post-natal rat lens almost all of αB-crystallin is associated with the Golgi membrane fraction. Immunolocalization, coupled with confocal microscopy, reveals a polar location for αB-crystallin in the epithelium; this location abuts the apical interface between fiber cells and the overlying epithelium. Of special note is the presence of αB-crystallin in perinuclear Golgi streaks in differentiating lens fiber cells, which contain elongating, degenerating nuclei. This association is sensitive to Brefeldin A, a fungal metabolite, which disorganizes Golgi. Comparisons of the distribution of αB-crystallin in two membrane fractions i.e., the vesicular and the Golgi fractions reveal that the vesicular: Golgi ratio increases appreciably from fetal to post natal life suggesting a possible involvement of Golgi -associated proteins and processes in fiber cell growth and maturation.
(a) Much against the perception that lens is ‘a crystallin-crammed bag’, the location of αB-crystallin (and therefore, the location of Golgi) is compartmentalized in the lens epithelium; in differentiating fiber cells αB-crystallin is present in Golgi streaks that follow the nuclear morphology. (b) In the early developing lens αB-crystallin must be regarded as a membrane associated protein.
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