Regulatory perspective on translating proteomic biomarkers to clinical diagnostics.

Issues associated with the translation of complex proteomic biomarkers from discovery to clinical diagnostics have been widely discussed among academic researchers, government agencies, as well as assay and instrumentation manufacturers. Here, we provide an overview of the regulatory framework and type of information that is typically required in order to evaluate in vitro diagnostic tests regulated by the Office of In Vitro Diagnostic Device Evaluation and Safety (OIVD) at the US Food and Drug Administration (FDA), with the focus on some of the issues specific to protein-based complex tests. Technological points pertaining to mass spectrometry platforms and assessment of potential concerns important for assurance of safety and effectiveness of this type of assays when introduced into clinical diagnostic use, as well as general approaches for evaluating the performance of these devices, are discussed.

The internalization of yeast Ste6p follows an ordered series of events involving phosphorylation, ubiquitination, recognition and endocytosis.

A general pathway for the internalization of plasma membrane proteins that involves phosphorylation, ubiquitination, recognition and endocytosis has recently emerged from multiple studies in yeast. We refer to this series of events as the PURE pathway. Here we investigate whether the yeast a-factor transporter Ste6p, an ATP-binding cassette protein, utilizes the PURE pathway. Deletion of a 52-amino acid sequence (the 'A box') within the linker region of Ste6p has previously been shown to block ubiquitination and endocytosis (Kolling R, Losko S. EMBO J 1997; 16:2251-2261). Using wild-type and mutant forms of GFP-tagged Ste6p, we identified two residues (T(613) and S(623)) within the A box as likely sites of Ste6p phosphorylation important for internalization. Mutation of these residues to alanine blocked ubiquitination and endocytosis of Ste6p, similar to the effect of deleting the entire A box, while substitution with glutamic acid (to mimic phosphorylation) suppressed the ubiquitination and endocytic defects. Importantly, a translational fusion of monoubiquitin to the C-terminus of Ste6p-T(613)A, S(623)A or Ste6p-DeltaA restored endocytosis, providing strong evidence that the role of phosphorylation is to direct ubiquitination, which in turn is a critical signal for Ste6p internalization. We also identified multiple (five) lysine residues in the linker that are important for Ste6p ubiquitination. Our results demonstrate that Ste6p follows the PURE pathway and that GFP-tagged Ste6p provides a powerful model protein for studies of endocytosis and post-endocytic events in yeast.