SUMMARYElectrostatic potentials along the ribosomal exit tunnel are non-uniform and negative. The significance of electrostatics in the tunnel remains relatively uninvestigated, yet is likely to play a role in translation and secondary folding of nascent peptides. To probe the role of nascent peptide charges in ribosome function, we used a molecular tape measure that was engineered to contain different numbers of charged amino acids localized to known regions of the tunnel, and measured chain elongation rates. Positively-charged arginine or lysine sequences produce transient arrest (pausing) before the nascent peptide is fully elongated. The rate of conversion from transiently arrested to full-length nascent peptide is faster for peptides containing neutral or negatively-charged residues than for those containing positively-charged residues. We provide experimental evidence that extra-ribosomal mechanisms do not account for this charge-specific pausing. We conclude that pausing is due to charge-specific interactions between the tunnel and the nascent peptide.

Helicity of membrane proteins can be manifested inside the ribosome tunnel, but the determinants of compact structure formation inside the tunnel are largely unexplored. Using an extended nascent peptide as a molecular tape measure of the ribosomal tunnel, we have previously demonstrated helix formation inside the tunnel. Here, we introduce a series of consecutive polyalanines into different regions of the tape measure to monitor the formation of compact structure in the nascent peptide. We find that the formation of compact structure of the polyalanine sequence depends on its location. Calculation of free energies for the equilibria between folded and unfolded nascent peptides in different regions of the tunnel shows that there are zones of secondary structure formation inside the ribosomal exit tunnel. These zones may have an active role in nascent-chain compaction.

Ionic fluxes are thought to be involved in mediating the proliferation of peripheral blood lymphocytes (PBLs) in response to mitogenic substances. Among the earliest events occurring after the addition of mitogen to cultured lymphocytes are changes in rates of cation transport. We were interested, therefore, in the possible role of ion channels in mediating the lymphocyte proliferative response. The development of patch clamp techniques by Neher and colleagues has made it possible to study membrane conductances in a variety of small cell types. We have developed a method which uses monoclonal antibodies to make cells adhere to solid surfaces for two major reasons: (1) it is much easier to patch clamp stationary cells, and (2) the method can be used to selectively adhere a particular cell type from a heterogeneous population. We have used these techniques here to identify whole-cell potassium currents, in lymphocytes recognized by OKT11 monoclonal antibody, which are increased 1.9-fold by mitogenic stimulation.

Transmembrane segments in the intact voltage-gated potassium (Kv) channel are helical. To ascertain whether this helicity could first be manifested inside the ribosomal tunnel, we generated biogenic peptide intermediates of Kv1.3 and mass-tagged the cysteine-scanned S6 transmembrane segment using pegylation (PEG-MAL) and calmodulation (CaM-MAL). For reference, we created an extended peptide that was used as a "molecular tape measure" of the ribosomal tunnel and determined that the functional length of the tunnel is 99-112 A. We demonstrate that the S6 segment forms a compact structure inside the ribosomal tunnel and that the N-terminal half of S6 compacts more than the C-terminal half of S6. These results bear on the earliest folding events during biogenesis of ion channels.

The lymphocyte voltage-gated K+ channel, Kv1.3, inactivates by a C-type process. We have elucidated the molecular basis for this process using a kinetic analysis of wild-type and mutant (A413V) Kv1.3 homo- and heteromultimeric currents in a mammalian lymphoid expression system. The medians of the measured inactivation time constants for wild-type and A413V homotetrameric currents are 204 and 4 ms, respectively. Co-expression of these subunits produces heteromultimeric channels manifesting inactivation kinetics intermediate between those of wild-type and A413V homomultimers. We have considered several models in which each subunit acts either independently or cooperatively to produce the observed inactivation kinetics. The cooperative model gives excellent fits to the data for any heteromultimeric composition of subunits, clearly distinguishing it from the independent models. In the cooperative model, the difference in free energy between the open and inactivated states of the channel is invariant with subunit composition and equals approximately 1.5 kcal/mol. Each subunit contributes equally to the activation free energy for transitions between open and inactivated states, with an A413V subunit decreasing the free energy barrier for inactivation (and for recovery from inactivation) by approximately 0.6 kcal/mol. Our results are consistent with a physical model in which the outer mouth of the channel constricts during C-type inactivation (G. Yellen, D. Sodickson, T. Chen, and M.E. Jurman, 1994, Biophys. J. 66:1068-1075).