Hsp90 as a capacitor for morphological evolution

Genetic analysis of viable Hsp90 alleles reveals a critical role in Drosophila spermatogenesis

The Hsp90 chaperone protein maintains the activities of a remarkable variety of signal transducers, but its most critical functions in the context of the whole organism are unknown. Point mutations of Hsp83 (the Drosophila Hsp90 gene) obtained in two different screens are lethal as homozygotes. We report that eight transheterozygous mutant combinations produce viable adults. All exhibit the same developmental defects: sterile males and sterile or weakly fertile females. We also report that scratch, a previously identified male-sterile mutation, is an allele of Hsp82 with a P-element insertion in the intron that reduces expression. Thus, it is a simple reduction in Hsp90 function, rather than possible altered functions in the point mutants, that leads to male sterility. As shown by light and electron microscopy, all stages of spermatogenesis involving microtubule function are affected, from early mitotic divisions to later stages of sperm maturation, individualization, and motility. Aberrant microtubules are prominent in yeast cells carrying mutations in HSP82 (the yeast Hsp90 gene), confirming that Hsp90 function is connected to microtubule dynamics and that this connection is highly conserved. A small fraction of Hsp90 copurifies with taxol-stabilized microtubule proteins in Drosophila embryo extracts, but Hsp90 does not remain associated with microtubules through repeated temperature-induced assembly and disassembly reactions. If the spermatogenesis phenotypes are due to defects in microtubule dynamics, we suggest these are indirect, reflecting a role for Hsp90 in maintaining critical signal transduction pathways and microtubule effectors, rather than a direct role in the assembly and disassembly of microtubules themselves.

Identification of SSF1, CNS1, and HCH1 as multicopy suppressors of a Saccharomyces cerevisiae Hsp90 loss-of-function mutation

Hsp90 functions in a multicomponent chaperone system to promote the maturation and maintenance of a diverse, but specific, set of target proteins that play key roles in the regulation of cell growth and development. To identify additional components of the Hsp90 chaperone system and its targets, we searched for multicopy suppressors of various temperature-sensitive mutations in the yeast Hsp90 gene, HSP82. Three suppressors were isolated for one Hsp90 mutant (glutamate --> lysine at amino acid 381). Each exhibited a unique, allele-specific pattern of suppression with other Hsp90 mutants and had unique structural and biological properties. SSF1 is a member of an essential gene family and functions in the response to mating pheromones. CNS1 is an essential gene that encodes a component of the Hsp90 chaperone machinery. The role of HCH1 is unknown; its sequence has no strong homology to any protein of known function. SSF1 and CNS1 were weak suppressors, whereas HCH1 restored wild-type growth rates at all temperatures tested to cells expressing the E381K mutant. Overexpression of CNS1 or HCH1, but not SSF1, enhanced the maturation of a heterologous Hsp90 target protein, p60(v-src). These results suggest that like Cns1p, Hch1p is a general modulator of Hsp90 chaperone functions, whereas Ssf1p likely is either an Hsp90 target protein or functions in the same pathway as an Hsp90 target protein.

Homeobox genes, fossils, and the origin of species

Ever since Darwin there has been a history of debate on the tempo and mode of evolution. Is speciation a gradual process involving the accumulation of minute variations extant within a species, or is it rapid, the result of major organismal reorganization? Does one define a species on the basis of genes, morphology, or geographic or reproductive isolation? In this communication I present a model of evolutionary change that is based on the Mendelian inheritance of mutations in regulatory genes and the fact that most nonlethal mutations arise in the recessive state. Since the new recessive allele will spread through many generations without expression until there is a critical mass of heterozygotes capable of producing homozygotes for the mutation, the novel feature thus produced will appear abruptly in the population and in more than one individual. This picture of punctuation is consistent with the fossil record, which typically fails to provide evidence of smoothly transitional states of morphological change. Given that the first of their kind in the fossil record are organisms in which their novel characteristics are often more fully expressed or complex than in their descendants, it would seem that, after the mutation involving a regulatory gene is introduced, the general tendency is for its effects to become diminished. Among the implications for speciation is that this process does not depend on either reproductive isolation or genetic incompatibility. Rather, barring effects on reproductive organs or behavior, homozygotes for a novelty should be able to breed with heterozygotes and homozygotes for the wild state of the original population. This, in turn, suggests that the species barrier between individuals is probably a matter of mate recognition.

Insect evolution: Redesigning the fruitfly

Homeotic mutations in Drosophila can result in dramatic phenotypes that suggest the possibility for rapid morphological evolution, but dissection of the genetic pathway downstream of Ultrabithorax is beginning to reveal how wing morphology may have evolved by more gradual transformations.

Reappraisal of the role of heat shock proteins as regulators of steroid receptor activity

Almost 30 years have passed since the original demonstration that steroid receptors, comprising a subfamily of the nuclear receptor (NR) superfamily, exist as large (6-8S) non-DNA-binding complexes in hypotonic extracts (cytosol) of target cells; later such complexes were shown to correspond to a heterooligomer composed of receptor, heat shock (Hsp), and other proteins. Subsequently, an impressive number of studies have dealt with the composition of the "nonactive" complex, its dissociation and/or reassembly in vitro, possible functions of the non-receptor components, and their subcellular compartmentalization. While there is little dispute about the chaperoning role of some Hsps in such a complex, there is still no final proof of an association in vivo of NRs and Hsps in the nuclei of target cells, which is requisite for a direct regulatory involvement of Hsps in NR function. Here we critically review the various models that have been put forward to attribute a biological function to the NR-Hsp90 interaction, evaluate the corresponding experimental data, and integrate recent concepts originating from the structural and functional analyses of NRs.