Is there a Population II analogy to the F star lithium dip?

Observers have found a small number of lithium-depleted halo stars in the temperature range of the Spite plateau. The current status of the mass-loss hypothesis for producing the observed lithium dip in Population (Pop) I stars is briefly discussed and extended to Pop II stars as a possible explanation for these halo objects. Based on detections of F-type main-sequence variables, mass loss is assumed to occur in a narrow temperature region corresponding to this “instability strip.” As Pop II main-sequence stars evolve to the blue, they enter this narrow temperature region, then move back through the lower temperature area of the Spite plateau. If 0.05 M⊙ (solar mass) or more have been lost, they will show lithium depletion. This hypothesis affects the lithium-to- beryllium abundance, the ratio of high- to low-lithium stars, and the luminosity function. Constraints on the mass-loss hypothesis due to these effects are discussed. Finally, mass loss in this temperature range would operate in stars near the turnoff of metal-poor globular clusters, resulting in apparent ages 2 to 3 Gyr (gigayears) older than they actually are.

DIP: The Database of Interacting Proteins: 2001 update

The Database of Interacting Proteins (DIP; http://dip.doe-mbi.ucla.edu) is a database that documents experimentally determined protein–protein interactions. Since January 2000 the number of protein–protein interactions in DIP has nearly tripled to 3472 and the number of proteins to 2659. New interactive tools have been developed to aid in the visualization, navigation and study of networks of protein interactions.

Extraordinary divergence and positive Darwinian selection in a fusagenic protein coating the acrosomal process of abalone spermatozoa.

During fertilization in marine invertebrates, fusion between sperm and egg cell membranes occurs at the tip of the sperm acrosomal process. In abalone sperm the acrosomal process is coated with an 18-kDa protein. In situ, this protein has no effect on the egg vitelline envelope, but in vitro it is a potent fusagen of liposomes. Thus, the 18-kDa protein may mediate membrane fusion between the gametes, a step in gamete recognition known to restrict heterospecific fertilization in other species. The cDNA and deduced amino acid sequences of the 18-kDa protein were determined for five species of California abalone. The deduced amino acid sequences exhibit extraordinary divergence; the percent identity varies from 27% to 87%. Analysis of nucleotide substitution shows extremely high frequencies of amino acid-altering substitution compared to silent substitution, demonstrating that positive Darwinian selection promotes the divergence of this protein. However, amino acid replacement is conservative with respect to size and polarity of residue. The data support the developing idea that in free-spawning marine invertebrates, the proteins mediating fertilization may be subjected to intense, and as yet unknown, selective forces. The extraordinary divergence of fertilization proteins may be related to the establishment of barriers to heterospecific fertilization.