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P rotein-based inheritance and prions are discussed relative to Saccharomyces and Podospora spp. On the basis of genetic properties two non-Mendelian genetic elements of S. cerevisiae, called (URE3) and (PSI), are prposed to be prions, or infectious proteins of Ure2p and Sup35p, respectively. Topics include genetic criteria for a prion, a prion form of Ure2p affecting nitrogen catabolism, a prion form of Sup35p, a translation release factor, (Het-s)Hets prion control of vegetative incompatibility in Podospora, comparison of prion systems, and structure-based inheritance: cortical inheritance on Paramecium and other ciliates.

The products of the Saccharomyces cerevisiae CIN1, CIN2 and CIN4 genes participate in a nonessential pathway required for normal microtubule function. In this article, we demonstrate that the product of PAC2 also functions in this pathway. PAC2 deletion mutants displayed phenotypes and genetic interactions similar to those caused by cin1[Delta], cin2[Delta] and cin4[Delta]. These include cold-sensitive microtubule structures and sensitivity to the microtubule depolymerizing agent benomyl. Involvement in a common functional pathway is indicated by the observation that all double mutant combinations are viable and no more affected than any single mutant. In addition, extra copies of CIN1 were found to suppress the benomyl sensitivity of pac2[Delta], cin2[Delta] and cin4[Delta], but not that caused by other mutations that affect microtubule function. Cin1p and Pac2p were found to be related in sequence to mammalian proteins that aid in the folding of [Beta]-tubulin into an assembly-competent state. Alleles of CIN1 were identified that could suppress the benomyl sensitivity of cin4-4 in a highly specific fashion. Our findings suggest that the guanine nucleotide-binding Cin4p interacts with Cin1p and regulates its tubulin folding activity.

Prions are infectious proteins. [[PIN.sup.+]] is a non-chromosomal genetic element of Saccharomyces cerevisiae that is necessary for the de novo induction of the [[PSI.sup.+]] prion. Recently, [[PIN.sup.+]] has been found to be itself a prion of the Rnq1 protein. [URE3], another yeast prion, can also promote [[PSI.sup.+]] generation. Thus, one prion can promote the generation of another.

The term Prion means 'infectious protein' in any organism, irrespective of the mechanism and implies the absence of a nuclei acid necessary for the infectivity. Infectious proteins (Prion) include the transmissible spongiform encephalopathies (TSES) of mammals, the amyloidoses [URE3], [PSI] and [PIN] of Saccharomyces cerevisiac, [Het-s] of the filamentous fungus podospora anserina, and the self-activating in trans vocular protease B of yeast.

Research has been conducted on infectious proteins prions. The authors demonstrate that yeast protease B is a prion, which is infectious and reversibly curable, and which is induced via pro-protease overexpression.

A cytoplasmic activity in mature oocytes responsible for second meiotic metaphase arrest was identified over 30 years ago in amphibian oocytes. In Xenopus oocytes cytostatic factor (CSF) activity is initiated by the progesterone-dependent synthesis of Mos, a MAPK kinase kinase that activates the MAPK pathway. CSF arrest is mediated by a sole MAPK target, the protein kinase p90Rsk. Rsk phosphorylates and activates the Bub1 protein kinase, which may cause metaphase arrest due to inhibition of the anaphase-promoting complex (APC) by a conserved mechanism defined genetically in yeast and mammalian cells. CSF arrest in vertebrate oocytes by p90Rsk provides a link between the MAPK pathway and the spindle assembly checkpoint in the cell cycle. [Copyright &y& Elsevier]

[URE3] and [PSI(+)] are infectious protein forms of the Saccharomyces cerevisiae Ure2p and Sup35p, respectively. We isolated an allele of SSA2, the primary cytosolic Hsp70, in a screen for mutants unable to maintain [URE3]. Designated ssa2-10, the mutation results in a leucine substitution for proline 395, a conserved residue of the peptide-binding domain. This allele also unexpectedly destabilizes [URE3] in newly formed heterozygotes: [URE3] is either absent in heterozygotes formed by crossing wild-type [URE3] cells with ssa2-10 mutants, or present and fully stable. SSA2 deletion mutants are weakly capable of maintaining [URE3]. The ssa2-10 allele is compatible with propagation of [PSI(+)]. However, in combination with a deletion of SSA1, ssa2-10 eliminates the nonsense-suppression phenotype of [PSI(+)] cells., (Copyright 2003 John Wiley & Sons, Ltd.)