Background: FKBP38 regulates the biogenesis of plasma membrane ion channels. Results: FKBP38 inhibits protein synthesis through its membrane anchorage and promotes CFTR post-translational folding through its PPIase domain, both negatively regulated by Hsp90 through the tetratricopeptide repeat domain. Conclusion: FKBP38 PPIase plays an important role in CFTR biogenesis. Significance: Our findings demonstrate an independent contribution of FKBP38 to CFTR biogenesis.

SIN2V is an engineered mutant Sindbis virus (SIN) that is unable to process the P23 cleavage site in polyproteins P123 and P1234 that are translated from the genome after its entry into cells. Unlike wild-type (wt) SIN, it caused minus strands to be made continuously and replication-transcription complex (RTC) activity to be unstable (R. Gorchakov, E. Frolova, S. Sawicki, S. Atasheva, D. Sawicki, and I. Frolov, J. Virol. 82:6218-6231, 2008). We examined further the effects of P23 on SIN RNA replication and RTC activity. Continuous minus-strand synthesis by SIN2V produced 250% of wt levels of minus strands but accumulated only 110% of wt levels (0.39 pg, or 2.7 ؋ 10 4 molecules of double-stranded RNA per cell). Because SIN2V-infected cells accumulated only 40% of the minus strands that were made, cells must possess some process to limit RTC accumulation. The loss of activity by SIN2V RTC after translation was inhibited was stochastic and not due to their inherent instability, based on finding that activity was lost without the degradation of the minus-strand templates. In addition to their normal functions, P23 RTCs exhibited the novel phenotype of being unable to switch from making less to making more genomes than subgenomic 26S mRNA at late times during infections. Our results lend credence to the hypothesis that nsP2 (and possibly nsP3) possesses functions other than those needed solely for RTC activity and that it may also act with the host to regulate minus-strand synthesis and the stability of the RTC.Alphaviruses are enveloped plus-strand RNA viruses, members of the family Togaviridae, that replicate in and cycle between vertebrate hosts and mosquito vectors. Sindbis virus (SIN) is a prototype alphavirus whose genome is an mRNA of 11,703 nucleotides with a 5Ј type 0 cap structure and a 3Ј poly(A) tail (40). After entry, the wild-type (wt) SIN genome is translated into nonstructural proteins (nsP) P123 and P1234, which are autoproteolytically and sequentially cleaved to form mature nsP1, nsP2, nsP3, and nsP4 (reviewed in references 35 and 40). Sequential cleavage of the nonstructural polyproteins was proposed to play a role in the regulation of viral minusstrand and plus-strand syntheses and to be essential to form three different polymerase activities from the P1234 precursors (14,20,21,37,44). This model suggested that release of nsP4 activated the polymerase subunit, first for minus-strand synthesis (P123 plus nsP4), followed by nsP1 cleavage that formed enzymes active in both genome plus-strand and minus-strand syntheses (nsP1, P23, and nsP4) but not active, or less active, in 26S mRNA synthesis and, finally, for P23 cleavage to release nsP2 and nsP3, a mature (nsP1 to -4) polymerase or replication-transcription complex (RTC) that efficiently recognizes the internal promoter for 26S mRNA synthesis while retaining the ability to produce genomes. Such a scheme provided explanations both for the short-lived nature of the minus-strand polymerase (33, 34) as being due to loss of promoter recognition with cle...

The cellular chaperone machinery regulates the biogenesis of nascent ion channels such as the cystic fibrosis transmembrane conductance regulator (CFTR). FK506‐binding protein 38 (FKBP38), an immunophilin anchored to the endoplasmic reticulum (ER) membrane, was identified as a component of a chaperone network associated with CFTR in the ER. Like FKBP52, FKBP38 binds Hsp90 through its tetratricopeptide repeat (TPR) domain. FKBP52 serves as an Hsp90 cochaperone in the activation of steroid receptors. However, the functional relationship between FKBP38 and Hsp90 in the context of CFTR biogenesis remains unclear. To understand the regulation of CFTR biogenesis by FKBP38, we introduced a series of domain‐specific, shRNA‐refractory mutants of FKBP38 into an FKBP38‐deficient cell line. Our data support a central role for the peptidylprolyl cis/trans isomerase in FKBP38‐mediated folding of CFTR and a moderate regulatory role for Hsp90 in FKBP38‐mediated folding and/or quality control of CFTR in the ER.