One size does not fit all: the oligomeric states of αB crystallin

Abstract

Small Heat Shock Proteins (sHSPs) are a diverse family of molecular chaperones that delay protein aggregation through interactions with non-native and aggregate-prone protein states. This function has been shown to be important to cellular viability and sHSP function/dysfunction is implicated in many diseases, including Alzheimer's and Alexander disease. Though their gene products are small, many sHSPs assemble into a distribution of large oligomeric states that undergo dynamic subunit exchange. These inherent properties present significant experimental challenges for characterizing sHSP oligomers. Of the human sHSPs, αB crystallin is a paradigm example of sHSP oligomeric properties. Advances in our understanding of sHSP structure, oligomeric distribution, and dynamics have prompted the proposal of several models for the oligomeric states of αB. The aim of this review is to highlight characteristics of αB crystallin (αB) that are key to understanding its structure and function. The current state of knowledge, existing models, and outstanding questions that remain to be addressed are presented.

Figure 1

αBdomain architecture, primary and secondary structure. The three domains/regions that define sHSPs are highlighted on the sequence of αB, with NTR residues in blue text, ACD residues in green, and CTR residues in red. Every tenth residue is underlined as a point of reference. In the NTR, three known phosphorylation sites are shown in cyan. The IXI/V motif in the CTR is shown in the black box. Secondary structure is summarized in three rows above the sequence, as cylinders (α-helix) and arrows (βstrand). TOP (magenta): secondary structure in the Braun model (PDB ID 2ygd). MIDDLE (black): secondary structure in the Jehle model. BOTTOM: experimentally-derived structural information, with secondary structure determined from ssNMR-based ACD structure shown in light blue, structure inferred from sparse ssNMR restraints shown in grey, and flexible regions measured by ssNMR and solution based NMR are shown in open circles.

Figure 2

ssNMR structure of the αB ACD dimer (PDB 2klr) with the CTR IXI/V motif bound. Residues involved in intermolecular contacts are highlighted on both protomers. Regions involved in intra-dimer contacts that define the dimer interface are shown in blue and the β4/β8 grove involved in inter-dimer interactions with the CTR IXI/V motif (from a neighboring dimer) are shown in red. The CTR sequence PERTIPITREEK is shown in black (bound to one protomer for clarity) and the Iles of the IXI/V motif are rendered as sticks. Two views of the ACD are presented, rotated 180° about the x-axis.

Figure 3

Comparison of the two pseudo-atomic models of an αB 24mer. The NTR (Blue), ACD (Green), and CTR (red) are displayed in different colors in the Jehle model (top row) and the Braun model (bottom row). The two models are aligned in the middle row, with the Jehle model in cyan and the Braun model in black. The hierarchy of assembly for the 24mer models is represented across columns A-D. The dimeric building block defined by intra-dimer contacts (A) are shown assembled into hexameric rings through CTR/ACD inter dimer interactions (B) and four hexamers are assembled through NTR interactions to form the 24mer oligomer (C and D). The models differ significantly in the placement of the NTR. The view in column D (rotated 55° about the y-axis relative to column C) clearly shows the Braun model to be a hollow particle with the NTR on the surface while the core of the particle is filled with the NTR in the Jehle model.