Loops in the central channel of ClpA chaperone mediate protein binding, unfolding, and translocation

The cylindrical Hsp100 chaperone ClpA mediates ATP-dependent unfolding of substrate proteins bearing "tag" sequences, such as the 11-residue ssrA sequence appended to proteins translationally stalled at ribosomes. Unfolding is coupled to translocation through a central channel into the associating protease, ClpP. To explore the topology and mechanism of ClpA action, we carried out chemical crosslinking and functional studies. Whereas a tag from RepA protein crosslinked proximally to the flexible N domains, the ssrA sequence in GFP-ssrA crosslinked distally in the channel to a segment of the distal ATPase domain (D2). Single substitutions placed in this D2 loop, and also in two apparently cooperating proximal (D1) loops, abolished binding of ssrA substrates and unfolded proteins lacking tags and blocked unfolding of GFP-RepA. Additionally, a substitution adjoining the D2 loop allowed binding of ssrA proteins but impaired their translocation. This loop, as in homologous nucleic-acid translocases, may bind substrates proximally and, coupled with ATP hydrolysis, translocate them distally, exerting mechanical force that mediates unfolding.     

ClpS, a substrate modulator of the ClpAP machine

In the bacterial cytosol, ATP-dependent protein degradation is performed by several different chaperone-protease pairs, including ClpAP. The mechanism by which these machines specifically recognize substrates remains unclear. Here, we report the identification of a ClpA cofactor from Escherichia coli, ClpS, which directly influences the ClpAP machine by binding to the N-terminal domain of the chaperone ClpA. The degradation of ClpAP substrates, both SsrA-tagged proteins and ClpA itself, is specifically inhibited by ClpS. In contrast, ClpS enhanced ClpA recognition of two heat-aggregated proteins in vitro and, consequently, the ClpAP-mediated disaggregation and degradation of these substrates. We conclude that ClpS modifies ClpA substrate specificity, potentially redirecting degradation by ClpAP toward aggregated proteins.     

Syntheses and stabilities of proteins related to the polyoma small T antigen in Escherichia coli.

We compared the syntheses and turnovers of two proteins related to the polyoma small T antigen synthesized in Escherichia coli from plasmids containing polyoma genomic segments joined to lac control elements. A protein with an authentic polyoma N terminus was more unstable than a protein with N-terminal amino acids derived from beta-galactosidase. Both were more unstable than most bacterial proteins.     

Development of behaviors in a male spectacled langur (Presbytis obscurus)

An infant langur,Presbytis obscurus, was observed in a group setting, from birth until 1 year of age. Frequency or duration of 30 behaviors was recorded during 2 hour periods and plotted chronologically. Based on fluctuations and interrelations of these behaviors, age classes were designated. The Maternal Care Period (0–20 days) is characterized by close mother-infant contact, including a great deal of nipple contact and a high frequency of maternal behaviors. The Individuation Period (21–70 days) is typified by maternal restraint and retrieval, and independence and self-oriented behaviors like scratching, mouthing, and locomotory skills. The Socialization Period (71–240 days) is manifested by behavioral fluctuations, involving nipple contact, play, and care by other troop members and coincides with the molt from infant to juvenile pelage. Finally, during the Juvenile Period, the motherinfant distance increases and the mother interacts more with other troop members. Behavioral fluctuations observed in this infant langur are discussed with reference to other primates studied.     

Feeding ecology of the black howler monkey (Alouatta pigra) in Northern Belize

We studied the feeding ecology of the black howler monkey (Alouatta pigra) from March 1994 to April 1995 in the Community Baboon Sanctuary in northern Belize, Central America. Activity and diet composition were recorded using continuous focal animal sampling. Diet composition was compared with the relative abundance of plant parts eaten by the howlers within the study site. The study animals spent an average of 24.4% of their time feeding, 61.9% resting, and approximately 9.8% traveling. In contrast to previously published reports on A. pigra, we found the diet composition to be similar to that of other Alouatta species (conforming to the folivore/frugivore profile), with 41% of feeding time spent eating fruit, 45% foliage, and 11% flowers. This contrast may indicate a wide degree of dietary flexibility that allows A. pigra to inhabit a variety of habitat types. We suggest that a high level of resource abundance throughout the year makes the Community Baboon Sanctuary excellent habitat for Alouatta pigra. Am. J. Primatol. 45:263–279, 1998. © 1998 Wiley-Liss, Inc.     

The GroEL/GroES cis cavity as a passive anti-aggregation device

The GroEL/GroES chaperonin folding chamber is an encapsulated space of ∼65 Å diameter with a hydrophilic wall, inside of which many cellular proteins reach the native state. The question of whether the cavity wall actively directs folding reactions or is playing a passive role has been open. We review past and recent observations and conclude that the chamber functions as a passive “Anfinsen cage” that prevents folding monomers from multimolecular aggregation.     

Chaperonin-mediated Protein Folding

We have been studying chaperonins these past twenty years through an initial discovery of an action in protein folding, analysis of structure, and elucidation of mechanism. Some of the highlights of these studies were presented recently upon sharing the honor of the 2013 Herbert Tabor Award with my early collaborator, Ulrich Hartl, at the annual meeting of the American Society for Biochemistry and Molecular Biology in Boston. Here, some of the major findings are recounted, particularly recognizing my collaborators, describing how I met them and how our great times together propelled our thinking and experiments.