SerpinB5, also known as maspin, is considered a tumor suppressor serpin that does not present itself as a protein inhibitor like others of its own family, the serine protease inhibitor superfamily (serpins). Maspin was first identified in 1994 on mammary tissue and breast cancer cell lines (1), but it is also known to be expressed on a wide range of cell types and tissues, mainly in epithelial cells, i. e. in prostate, lung, skin, and corneal stromal cells (2). It differs from ordinary serpins once it does not undergo the stressed (S) to relaxed (R) conformation which is a striking feature of other proteins in serpin’s superfamily. Instead, its G-helix has quite a flexibility, capable of changing the conformation of the protein itself.

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Maspin and its superfamily

Serpins

Serpins usually inhibit other proteins like serine proteases, caspases and papain-like cysteine proteases, however, some of them do not accomplish an inhibitory role (3). As an example, some of them function as hormone transporters, molecular chaperones or even as tumor suppressors (3). Serpins structure usually contain three ß-sheets (A, B and C) and eight to nine 𝛂-helices (hA-hI) on their structure, and the most important region to interact with their targets is the reactive center loop (RCL) (An overview of the serpin superfamily). Inhibitory serpins are considered “suicide molecules” because they can only be used once (Huntington, J., Read, R. & Carrell, R. Structure of a serpin–protease complex shows inhibition by deformation . Nature 407, 923–926 (2000). https://doi.org/10.1038/35038119). The RCL is usually positioned out of the body of the serpins. When inhibiting proteases, serpins get their RCL cleaved out of the main structure, causing the amino-terminal portion of the RCL to form an additional fourth strand called s4A, once it is inserted into the center of ß-sheet A. This cleavage and modification on the structure of serpin is called the ‘stressed (S) to relaxed (R) transition’, in which the protein is in its biologically active state and transitions to a more thermal stable and latent state, respectively (An overview of the serpin superfamily).

Function

Disease

Relevance

Structural highlights

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