The UDP family all contain a domain similar to ubiquitin but they are not conjugated to protein targets. The UBL/ULM family of proteins are used to modify protein substrates in a manner similar to that of ubiquitin as described below. The UBL/ULM family of proteins represents one of two classes of the ubiquitin-like molecules while the second class is composed of proteins that contain a ubiquitin domain and are, therefore, referred to as ubiquitin-domain proteins (UDP). The other members of the ubiquitin protein family are referred to as ubiquitin-like proteins or ubiquitin-like modifiers (or ubiquitin-like molecules) and given the designation, UBL and also often designated ULM. The RPS27A gene generates multiple alternatively spliced mRNAs that encode precursor proteins containing ubiquitin in the N-terminal part fused to small ribosomal subunit protein S27a in the C-terminal part. The RPS27A gene encodes a ubiquitin fusion protein like that encoded by the UBA52 gene. The UBA52 mRNAs encode proteins that contain ubiquitin at the N-terminal part of the fusion protein and large ribosomal subunit protein L40 at the C-terminal part. The UBA52 gene generates multiple alternatively spliced mRNAs that encode precursor ubiquitin fusion proteins. Like the UBB gene, the UBC gene generates an mRNA that encodes a polyubiquitin precursor protein. 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 TurnoverÄuring 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.
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