Lac repressor with the helix-turn-helix motif of lambda cro binds to lac operator.

Lac repressor, lambda cro protein and their operator complexes are structurally, biochemically and genetically well analysed. Both proteins contain a helix-turn-helix (HTH) motif which they use to bind specifically to their operators. The DNA sequences 5'-GTGA-3' and 5'-TCAC-3' recognized in palindromic lac operator are the same as in lambda operator but their order is inverted form head to head to tail to tail. Different modes of aggregation of the monomers of the two proteins determine the different arrangements of the HTH motifs. Here we show that the HTH motif of lambda cro protein can replace the HTH motif of Lac repressor without changing its specificity. Such hybrid Lac repressor is unstable. It binds in vitro more weakly than Lac repressor but with the same specificity to ideal lac operator. It does not bind to consensus lambda operator.     

Implications of a non-lamellar lipid phase for the tight junction stability. Part II: Reversible modulation of transepithelial resistance in high and low resistance MDCK-cells by basic amino acids, Ca2+, protamine and protons.

The transepithelial resistance of confluent epithelial cell monolayers was monitored to investigate the influence of basic amino acids, Ca2+, protamine and protons on tight junction electrical resistance. In an accompanying paper we investigated the effect of these substances on the lamellar/hexagonal II phase transition in reconstituted phospholipid membranes containing phosphatidylserine and phosphatidylethanolamine. We conclude that the permeability of tight junctions may be described by a lipid phase equilibrium where the lamellar phase corresponds to an open state and the hexagonal lipid phase to the closed state of the cell contact. This dynamic lipid model is well suited to describe the morphological as well as functional properties of the tight junctions.     

The (in)dependence of alternative splicing and gene duplication

Alternative splicing (AS) and gene duplication (GD) both are processes that diversify the protein repertoire. Recent examples have shown that sequence changes introduced by AS may be comparable to those introduced by GD. In addition, the two processes are inversely correlated at the genomic scale: large gene families are depleted in splice variants and vice versa. All together, these data strongly suggest that both phenomena result in interchangeability between their effects. Here, we tested the extent to which this applies with respect to various protein characteristics. The amounts of AS and GD per gene are anticorrelated even when accounting for different gene functions or degrees of sequence divergence. In contrast, the two processes appear to be independent in their influence on variation in mRNA expression. Further, we conducted a detailed comparison of the effect of sequence changes in both alternative splice variants and gene duplicates on protein structure, in particular the size, location, and types of sequence substitutions and insertions/deletions. We find that, in general, alternative splicing affects protein sequence and structure in a more drastic way than gene duplication and subsequent divergence. Our results reveal an interesting paradox between the anticorrelation of AS and GD at the genomic level, and their impact at the protein level, which shows little or no equivalence in terms of effects on protein sequence, structure, and function. We discuss possible explanations that relate to the order of appearance of AS and GD in a gene family, and to the selection pressure imposed by the environment.     

Genome-wide CRISPR Screens in T Helper Cells Reveal Pervasive Crosstalk between Activation and Differentiation

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