The UBB gene generates multiple alternatively spliced mRNAs that encode proteins that are composed of three direct repeats of the ubiquitin coding sequence and, therefore, produce a polyubiquitin precursor protein. Humans express four different genes that produce the protein ubiquitin, UBB, UBC, UBA52, and RPS27A (also known as UBA80). The originally characterized function of ubiquitin was its attachment to proteins to target them for degradation in the protein degradation apparatus termed the proteasome. The protein ubiquitin is the founding member of a family of proteins that includes at least 20 members all of which are involved in post-translational modifications of numerous substrate proteins. Ubiquitin and Protein Ubiquitylation (Ubiquitination) However, certain details of protein turnover, involving the ubiquitin and ubiquitin-like modifier pathways, and the pathways of organelle turnover, and the programmed cell death pathways will be highlighted. This page is not intended to be an exhaustive overview of the details of all of these macromolecule and cell turn-over processes. Ultimately, if these controlled and regulated processes become overwhelmed a program of cell death, called apoptosis, can be initiated in order to destroy aberrant cells as a means to ensure overall tissue survival. These processes are regulated in highly specific ways that include protein modification, protein degradation and amino acid recycling, organelle tagging, engulfment, and lysosomal degradation with component parts being recycled. The processes by which proteins and organelles are degraded, removed and even recycled are vital for cellular survival. In addition, cellular components such as proteins and even organelles can become damaged and the cell requires mechanisms to remove the damaged entities in order to ensure viability. Introduction to Processes of Protein, Organelle, and Cell Turnoverĭuring the normal course of cellular life macromolecules serve their intended purposes and must be degraded or turned over to ensure their function(s) remain tightly regulated. Perforin and Granzyme-Mediated Apoptosis.Table of Death Receptor Pathway Proteins.Endoplasmic Reticulum Stress-Mediated Apoptosis.Table of Pro-Apoptosis BCL-2 Family Proteins.Table of Pro-Survival BCL-2 Family Proteins.Table of Inhibitors of Apoptosis Proteins, IAP.Caspases: Inducers of the Apoptotic Cascades.ATG Proteins in Autophagosome Formation.Table of Mammalian Autophagy Related Protein Family.Table of Membrane Trafficking Proteins in Autophagosome Formation.Transcription Factor EB: Master Regulator of Autophagy.Table of Several Mammalian Substrates for SUMO.Ubiquitin and Protein Ubiquitylation (Ubiquitination).Introduction to Processes of Protein, Organelle, and Cell Turnover.

Protein turnover definition