Alpha-crystallin-type heat shock proteins: socializing minichaperones in the context of a multichaperone network

Abstract

Alpha-crystallins were originally recognized as proteins contributing to the transparency of the mammalian eye lens. Subsequently, they have been found in many, but not all, members of the Archaea, Bacteria, and Eucarya. Most members of the diverse alpha-crystallin family have four common structural and functional features: (i) a small monomeric molecular mass between 12 and 43 kDa; (ii) the formation of large oligomeric complexes; (iii) the presence of a moderately conserved central region, the so-called alpha-crystallin domain; and (iv) molecular chaperone activity. Since alpha-crystallins are induced by a temperature upshift in many organisms, they are often referred to as small heat shock proteins (sHsps) or, more accurately, alpha-Hsps. Alpha-crystallins are integrated into a highly flexible and synergistic multichaperone network evolved to secure protein quality control in the cell. Their chaperone activity is limited to the binding of unfolding intermediates in order to protect them from irreversible aggregation. Productive release and refolding of captured proteins into the native state requires close cooperation with other cellular chaperones. In addition, alpha-Hsps seem to play an important role in membrane stabilization. The review compiles information on the abundance, sequence conservation, regulation, structure, and function of alpha-Hsps with an emphasis on the microbial members of this chaperone family.

FIG. 1.

Simplified model of protein quality control under normal and heat shock conditions. (A) Under optimal growth conditions, the rate of transcription and translation is very high. As indicated by the thick arrow, most proteins fold spontaneously without the assistance of chaperones. Few proteins aggregate and are degraded by proteases. (B) After heat shock, the transcription and translation capacity is reduced. Temperature-induced unfolding returns previously folded proteins to the chaperone-dependent quality control system. Unfoldable proteins are removed by proteases. For simplicity, intermediate steps such as aggregation and disaggregation are not considered in this model.

FIG.2.

Amino acid alignment of bacterial and archaeal α-crystallins. Human αA- and αB-crystallins were added for comparison at the bottom. Abbreviations: Mjannas, M. jannaschii; Eco, E. coli; Vcholer, V. cholera; Ccr, C. crescentus; Mlo, M. loti; Bja, B. japonicum; Paerugi, P. aeruginosa; Avinela, A. vinelandii; Lpneumo, L. pneumoniae; Buchner, Buchnera sp. strain APS; Caceto, C. acetobutylicum; Ooeni, O. oeni; Mtu, M. tuberculosis; Xfastid, X. fastidiosa; Sth, S. thermophilus; Phoriko, P. horikoshii; Pabysii, P. abysii; Mleprae, M. leprae; Mintr; M. intracellulare; Maviu, M. avium; Salbus, S. albus; Synecho, Synechocystis sp. strain PCC 6803; Svulcan, S. vulcanus; Aaeolic, A. aeolicus; Tmarit, T. maritima; Afulg, A. fulgidus; Saurant, S. aurantiaca; Bha, B. halodurans; Dra, D. radiodurans; Mthermo, M. thermoautotrophicum; Ape, A. pernix; Tac, T. acidophilum; Hal, Halobacterium sp. NRC-1; Rprowaz, R. prowazekii; Bsu, B. subtilis; Homo-aA, Homo sapiens αA-crystallin; Homo-aB, Homo sapiens αB-crystallin. The initial alignment was constructed with CLUSTAL W (322) and then imported into the multiple sequence alignment editor and shading utility GeneDoc (www.psc.edu/biomed/genedoc) and further refined manually. White letters shaded in black or gray indicate amino acids that are identical in at least 80 or 60% of all proteins, respectively. The consensus sequence below the alignment lists these residues in capital and lowercase letters, respectively. Shaded residues printed in black are identical in at least 40% of all sequences. Structural features of M. jannaschii Hsp16.5 are depicted on top of the alignment according to crystal structure data (155). For comparison, see Fig. 5B. At the bottom, the secondary-structure assignment of human αA-crystallin is provided (162). α-Crystallin regions that were labeled by bis-ANS or related fluorescent probes are indicated in yellow (280). The highlighted M. jannaschii region is equivalent to the labeled peptide of pea Hsp18.1 (175). Segments that cross-linked to target proteins are shaded in blue (278, 281). Peptides identified in the labeling and cross-linking technique are represented in green.

FIG.2.

Amino acid alignment of bacterial and archaeal α-crystallins. Human αA- and αB-crystallins were added for comparison at the bottom. Abbreviations: Mjannas, M. jannaschii; Eco, E. coli; Vcholer, V. cholera; Ccr, C. crescentus; Mlo, M. loti; Bja, B. japonicum; Paerugi, P. aeruginosa; Avinela, A. vinelandii; Lpneumo, L. pneumoniae; Buchner, Buchnera sp. strain APS; Caceto, C. acetobutylicum; Ooeni, O. oeni; Mtu, M. tuberculosis; Xfastid, X. fastidiosa; Sth, S. thermophilus; Phoriko, P. horikoshii; Pabysii, P. abysii; Mleprae, M. leprae; Mintr; M. intracellulare; Maviu, M. avium; Salbus, S. albus; Synecho, Synechocystis sp. strain PCC 6803; Svulcan, S. vulcanus; Aaeolic, A. aeolicus; Tmarit, T. maritima; Afulg, A. fulgidus; Saurant, S. aurantiaca; Bha, B. halodurans; Dra, D. radiodurans; Mthermo, M. thermoautotrophicum; Ape, A. pernix; Tac, T. acidophilum; Hal, Halobacterium sp. NRC-1; Rprowaz, R. prowazekii; Bsu, B. subtilis; Homo-aA, Homo sapiens αA-crystallin; Homo-aB, Homo sapiens αB-crystallin. The initial alignment was constructed with CLUSTAL W (322) and then imported into the multiple sequence alignment editor and shading utility GeneDoc (www.psc.edu/biomed/genedoc) and further refined manually. White letters shaded in black or gray indicate amino acids that are identical in at least 80 or 60% of all proteins, respectively. The consensus sequence below the alignment lists these residues in capital and lowercase letters, respectively. Shaded residues printed in black are identical in at least 40% of all sequences. Structural features of M. jannaschii Hsp16.5 are depicted on top of the alignment according to crystal structure data (155). For comparison, see Fig. 5B. At the bottom, the secondary-structure assignment of human αA-crystallin is provided (162). α-Crystallin regions that were labeled by bis-ANS or related fluorescent probes are indicated in yellow (280). The highlighted M. jannaschii region is equivalent to the labeled peptide of pea Hsp18.1 (175). Segments that cross-linked to target proteins are shaded in blue (278, 281). Peptides identified in the labeling and cross-linking technique are represented in green.

FIG. 3.

Regulation of bacterial α-Hsp genes. A representative organisms for each regulatory strategy is listed. The regulated genes and corresponding regulators are indicated. Promoters are presented as −10 and −35 regions. Known repressor-binding sites occur as inverted repeats (IR) or directed repeats (DR). Details are presented in the text.

FIG. 4.

Postulated assembly pathways of α-Hsp complexes. Particles competent for binding unfolded proteins are shaded in gray. ΔT indicates a temperature upshift. For further details, see the text.

FIG. 5.

Overall structure and subunit interactions of Methanococcus jannaschii Hsp16.5. (A) In the space-filling model of the hollow sphere, each tetramer is represented in one color with different shadings. At the bottom, the interior of the sphere is viewed along the threefold axis (left) and the fourfold axis (right). The front one-third of each sphere is cut off. (B) Topology of the secondary structure of a Hsp16.5 dimer. The first and last residue numbers for each secondary-structure element are indicated in the left monomer, which is indicated by a dotted frame. The structural elements are labeled in the right monomer. (Reprinted from reference 155 with permission of the publisher.)

FIG. 6.

Function of α-Hsps in a multichaperone network. (A) Sequential transfer of α-Hsp-bound substrates for further processing. (B) Schematic representation of the cellular multichaperone network. For details, see the text.