![]() TLRs 1, 2 and 4–6 reside at the plasma membrane, where they detect molecules displayed on the surface of various pathogens. TLRs detect a wide range of microbial products, and may be divided into different groups based on their subcellular site of ligand recognition ( Barton and Kagan, 2009). SITES LIKE YOUTUNES PROFESSIONALTLRs are transmembrane receptors that are expressed by a variety of mammalian cell types, but are best studied in professional phagocytes such as macrophages and dendritic cells (DCs) ( Akira et al., 2006). An example of this can be found from the studies of the Toll-like Receptors (TLRs) of the innate immune system. In this latter instance, it is not obvious how a single localization signal would allow a protein to be targeted to (and function from) multiple organelles. In addition to these examples, a more complex type of protein movement exists, where proteins can be found in multiple locations under resting conditions, or can be recruited to multiple locations in response to a stimulus. Classic cell biological studies identified the cis-acting sequences that direct these types of protein movement within cells for example ( Blobel and Dobberstein, 1975a Blobel and Dobberstein, 1975b) and ( Baeuerle and Baltimore, 1988). Another type of movement is signal-dependent, meaning that the protein will move from one location to another in response to some cellular stimulus. Some movement is biosynthetic, in that the protein must be transported from its site of synthesis on the ribosome to its site of action. At some point in their existence, all proteins must move within mammalian cells. ![]()
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