Direct evidence for ligand-induced internalization of the yeast alpha-factor pheromone receptor.

When Saccharomyces cerevisiae a cells bind alpha-factor pheromone, the ligand is internalized and its binding sites are lost from the cell surface in a time-, energy-, and temperature-dependent manner. This report presents direct evidence for alpha-factor-induced internalization of cell surface receptors. First, membrane fractionation on Renografin density gradients indicated that the alpha-factor receptors were predominantly found in the plasma membrane peak before alpha-factor treatment and then appeared in membranes of lesser buoyant density after alpha-factor exposure. Second, receptors were susceptible to cleavage by extracellular proteases before alpha-factor treatment and then became resistant to proteolysis after exposure to pheromone, consistent with the transit of receptors from the cell surface to an internal compartment. The median transit time in both assays was approximately 8 min. The ultimate target of the internalized receptors was identified as the vacuole, since the membranes containing internalized receptors cofractionated with vacuolar membranes, since the turnover of receptors was stimulated by alpha-factor exposure, and since receptor degradation was blocked in a pep4 mutant that is deficient for vacuolar proteases. The carboxy-terminal domain of the receptor that is required for ligand internalization was also found to be essential for endocytosis of the receptor. A receptor mutant, ste2-L236H, which is defective for pheromone response but capable of ligand internalization, was found to be proficient for receptor endocytosis. Hence, separate structural features of the receptor appear to specify its signal transduction and internalization activities.

The C terminus of the Saccharomyces cerevisiae alpha-factor receptor contributes to the formation of preactivation complexes with its cognate G protein.

Binding of the alpha-factor pheromone to its G-protein-coupled receptor (encoded by STE2) activates the mating pathway in MATa yeast cells. To investigate whether specific interactions between the receptor and the G protein occur prior to ligand binding, we analyzed dominant-negative mutant receptors that compete with wild-type receptors for G proteins, and we analyzed the ability of receptors to suppress the constitutive signaling activity of mutant Galpha subunits in an alpha-factor-independent manner. Although the amino acid substitution L236H in the third intracellular loop of the receptor impairs G-protein activation, this substitution had no influence on the ability of the dominant-negative receptors to sequester G proteins or on the ability of receptors to suppress the GPA1-A345T mutant Galpha subunit. In contrast, removal of the cytoplasmic C-terminal domain of the receptor eliminated both of these activities even though the C-terminal domain is unnecessary for G-protein activation. Moreover, the alpha-factor-independent signaling activity of ste2-P258L mutant receptors was inhibited by the coexpression of wild-type receptors but not by coexpression of truncated receptors lacking the C-terminal domain. Deletion analysis suggested that the distal half of the C-terminal domain is critical for sequestration of G proteins. The C-terminal domain was also found to influence the affinity of the receptor for alpha-factor in cells lacking G proteins. These results suggest that the C-terminal cytoplasmic domain of the alpha-factor receptor, in addition to its role in receptor downregulation, promotes the formation of receptor-G-protein preactivation complexes.

Nucleotide sequence of the F plasmid gene, traC, and identification of its product.

The traC gene of the F plasmid tra operon is required for the assembly of mature F-pilin subunits into extended F pili. The nucleotide sequence of traC was determined with a determined with a deduced coding region of 875 amino acids (aa) and 99066 Da. The traC1044 mutant allele, which allows filamentous phage infection in the absence of piliation, contains a C-to-T transition leading to an Arg----Cys substitution. Confirmation of the translational start came from the direct N-terminal aa sequencing of a TraC-alkaline phosphatase fusion protein.

Localization of TraC, a protein involved in assembly of the F conjugative pilus.

TraC is one of the proteins encoded by the F transfer region of the F conjugative plasmid which is required for the assembly of F pilin into the mature F pilus structure. Overproduction of this protein from the plasmid pKAS2, which carries only traC, resulted in the formation of inclusion bodies from which soluble TraC was purified. When small amounts of TraC were produced from pKAS2, the protein was localized to the cytoplasm by using anti-TraC antibodies. Similar analysis of a set of TraC-alkaline phosphatase fusion proteins localized all of these fusion proteins to the cytoplasm. However, when TraC was expressed from the F plasmid, much of it appeared associated with the bacterial membrane fraction. Under these conditions, TraC does not appear to be part of the tip of the F pilus, as neither anti-TraC antibodies nor purified TraC had any effect on the infection of F-containing bacteria by the filamentous bacteriophage f1. These data suggest that TraC is normally associated with the membrane through interactions with other proteins specified by the tra region. This interaction may be via the carboxyl-terminal region of the TraC protein, as a mutant TraC protein containing an Arg-Cys substitution at amino acid 811 exhibits an interaction with the membrane weaker than that of the wild-type protein in the presence of the other Tra proteins.

A traC mutant that retains sensitivity to f1 bacteriophage but lacks F pili.

An F lac pro mutant which was temperature sensitive for infection by the filamentous bacteriophage f1 but resistant to the F-specific icosahedral RNA phage f2 was isolated. Cells carrying the F' mutation failed to elaborate F pili at all temperatures. Mutant cells were able to pair with recipient cells during bacterial conjugation, but transfer of conjugal DNA occurred at a greatly reduced frequency. Complementation analyses showed the F' mutation to be in the traC gene. When a plasmid carrying traC was introduced into hosts harboring the F' mutation, phage sensitivity, the ability to elaborate F pili, and conjugation efficiency were restored. The mutation was named traC1044. The F lac pro traC1044 mutant appears to be unique among traC mutants in retaining host sensitivity to the filamentous phage f1 in the absence of expression of extended F pili. Phage f1 attachment sites appeared to be present at the cell surface in traC1044 mutants. The reduced accessibility of these sites may account for the reduced efficiency of phage f1 infection of traC1044 hosts, although the possibility that a defect was present in the receptor site itself was not eliminated. Membranes of hosts carrying the F' mutation contained a full complement of mature F-pilin subunits, so the product of traC is presumably required for pilus assembly but not for pilin processing. This, together with the deficiency in conjugal DNA transfer, suggests that traC may be part of a membrane-spanning tra protein complex responsible for pilus assembly and disassembly and conjugal DNA transmission.