Interactions between Kar2p and Its Nucleotide Exchange Factors Sil1p and Lhs1p Are Mechanistically Distinct

Kar2p, an essential Hsp70 chaperone in the endoplasmic reticulum of Saccharomyces cerevisiae, facilitates the transport and folding of nascent polypeptides within the endoplasmic reticulum lumen. The chaperone activity of Kar2p is regulated by its intrinsic ATPase activity that can be stimulated by two different nucleotide exchange factors, namely Sil1p and Lhs1p. Here, we demonstrate that the binding requirements for Lhs1p are complex, requiring both the nucleotide binding domain plus the linker domain of Kar2p. In contrast, the IIB domain of Kar2p is sufficient for binding of Sil1p, and point mutations within IIB specifically blocked Sil1p-dependent activation while remaining competent for activation by Lhs1p. Taken together, these results demonstrate that the interactions between Kar2p and its two nucleotide exchange factors can be functionally resolved and are thus mechanistically distinct.     

An Ethometric Analysis of Agonistic Behavior and Social Hierarchies in the Cockroach,Eublaberus posticus

Agonistic behavior between adult ♂♂ of the tropical cockroach Eublaberus posticus was studied. Observations were made under red light (590–640 nm) during the dark phase of a 12: 12 DL cycle, and recorded on a dictation instrument. Agonistic behavior appears to be causally related to territoriality. Social hierarchies are established and maintained by agonistic behavior. The application of information theory to the data shows that more information is transferred from the responder (eventual loser) to the initiator (eventual winner) of an agonistic encounter than from the initiator to the responder. A measure of information called normalized transmission is calculated which can be used to compare different species.     

X-ray induced translocations in spermatogonia. I. Dose and fractionation responses in mice

The frequency of recirprocal translocations, inducedm by X-irradiation of mouse spermatogonial cells and observed at diakinesis-metaphase I in primary spermatocytes, was measured over a dose range of 0–1200 R. The resulting dose-response curve gave a best fit to the model Y = bD+CD² over the range of 0–500 R. Above 600 R, howeverm, the yeild of translocations decreased with increasing dose, leadiong to a “humped” dose-response curve over the whole dose range studied, as has been observed by several worker previously.The significance of the nonlinear dose-response curve over the lower dose range is discussed in terms of the known fractionation and dose-rate effects for reciprocal translocations induced in spermatogonia.A dose of 800 R was split into two 400-R fractions separated by 8 weeks, or one of 1200 R into three equal parts, each separated by an 8-week interval. The resulting yield of translocations was the same as the sum of two, or three, separate 400-dose doses, but was much higher than a single dose of 800 R or 1200 R.It is suggested that these results, namely the shape of the dose-response curve and the “reverse” fractionation effect, can be explained in terms of resistant and sensitive stem-cell populations, but that any one cell can be in either population, depending upon the stage of the cell cycle in which it is at the time of irradiation.     

Cellulose-microfibril-orienting mechanisms in plant cells walls

A brief review is given of the changing views over the years, as knowledge of wall structure has developed, concerning the mechanism whereby cellulose chains may be oriented. This leads to an examination of current concepts, particularly those concerning microtubules. It is shown that none of the mechanisms suggested whereby microtubules might cause orientation of cellulose microfibrils is consistent with the known range of molecular architectures found in plant cell walls. It is further concluded that any mechanism which necessitates an indissoluble link between the plasmalemma and the cellulose-synthesising complex at the tip of a microfibril is unacceptable. A new proposal is presented in which it is speculated that both microtubules and microfibrils are oriented by a mechanism separate from both. It is shown that if two vectors are contemplated, one parallel to cell length and one at right angles, and a sensor exists on the plasmalemma surface which responds to changes in the vectors, then all known wall structures may be explained. The possible nature of the vectors and the sensor are considered.