Though frequently overlooked as being a "catch-all" for proteins that don't reside within another organelle, cytoplasmic proteins are anything but.

One important role of proteins in the cytoplasm is the regulation of gene expression. There are two main ways in which gene expression is regulated; when converting the genetic code in your DNA to RNA (transcription), and when converting RNA to proteins (translation). Though transcription (DNA to RNA) occurs in the nucleus, cytoplasmic proteins are often involved in activating transcription factors which then move to the nucleus to perform transcription.

The protein stained in Fig 1. is an image of RELA in U-2 OS human osteosarcoma cells. RELA is a well known protein that binds NF-kB2 (Nuclear Factor kappa-light-chain-enhancer of activated B cells) in the cytoplasm and holds it in an inactive state. When signalled, RELA is degraded and NF-kB moves into the nucleus where its active form is key in activating the transcription of certain genes.

NF-kB is one of the most widely expressed and widely studied proteins and is highly conserved across animal evolution (Ghosh S, May MJ, & Kopp EB 1998). A major reason for this is that NF-kB is a key activator in inflammation and immune response (Hayden MS1, West AP, Ghosh S 2006). How and when RELA regulates NF-kB is a major topic of research, and understanding this system will be key in understanding immune response (Li X et al. 2014).

The cytoplasmic location of RELA, and subsequently NF-kB is crucial in preventing inappropriate inflammatory response. Dysregulation of this system can lead to several autoinflammatory diseases such as Crohn's disease or rheumatoid arthritis (Atreya I, Atreya R, Neurath MF 2008, PP Tak and GS Firestein 2001). Recently, NF-kB and its dysregulation has also been linked to both cancer initiation and progression (B Hoesel & JA Schmid 2013).

This image provides only a brief insight into just one important role of cytoplasmic proteins in your cells. Though they may not be the most visually interesting, never discount the importance of these proteins.

A big thanks to all the members of the subcellular atlas team working to generate these images!

Devin Sullivan