Cell signaling – the biochemical processes by which cells communicate, transfer, and process information – is vital for proper cellular functioning. Cellular fate and tissue-specific gene expression are regulated by the activity of proteins known as transcription factors that interact with specific DNA sequences to direct the activation or repression of target genes.
One of the simpler modes of signal transmission from surface receptors to the nucleus is illustrated by proteins that bind to DNA sequences known as cAMP-response elements (CREs), the proteins being called CRE-binding proteins (CREB proteins). A more complex mechanism involving the activation of protein kinase cascades is used to transmit signals from growth-factor receptors to the immediate-early genes that are regulated by transcription factors such as AP-1 (which is a heterodimer of c-jun and c-fos). The major function of these cascades is to activate members of the mitogen-activated protein kinase (MAPK) family of serine/threonine protein kinases, and downstream protein kinases. A third mechanism for transmitting signals from the cell surface to the nucleus – first demonstrated for NFkB – is regulated nuclear translocation of transcription factors that are stored as inactive cytoplasmic complexes. Certain transcription factors such as Smads, act as transcriptional activators or repressors.
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