Cells integrate information from a large number of extracellular and intracellular signals in order to elicit specific responses. As part of this integration, the stoichiometry, stability, and compartmentalization of signaling complexes are regulated by scaffold proteins. Located most often at branch points within signaling networks, scaffold proteins mediate crosstalk between key molecules and can potentiate or inhibit the strength of signals. Models of pathway connections through scaffold proteins vary from simple to complex. For example, a scaffold protein can function to facilitate interactions in a linear manner or it can mediate pathway branching and amplification of output to multiple downstream partners. Finally, scaffolds can be part of more elaborate interaction networks involving feedback regulation, thus directly activating or blocking signaling pathways. In turn, scaffold proteins can be regulated based on a cell’s needs and the presence or absence of scaffold proteins therefore contributes to the diversity of possible cellular responses.
Functional studies indicate a role for several scaffold proteins in breast cancer. For instance, modulator of non-genomic action of estrogen receptor (MNAR) mediates the interactions of ER with PI3K and Src, and is required for the rapid effects of estrogen in breast cancer cells. POSH is another scaffold protein the proapoptotic function of which is suppressed by AKT in breast cancer cells. The Gab2 adaptor/scaffold protein is required to promote mammary tumor metastasis in neu-expressing mice. A recent study investigated the GIP1 scaffold protein, which was previously linked to IGF-IR and other receptors. GIP1 gene silencing inhibited proliferation and induced apoptosis of breast cancer cells.
In conclusion, scaffold proteins are essential for complex signaling networks that impact various cellular functions. Changes in the levels of scaffold protein genes or their mislocalization can potentially alter signal transduction and lead to faulty cell decisions that may ultimately contribute to breast tumorigenesis. The following section addresses some of the currently known facts about the scaffold protein MP1 and its potential relevance in the context of breast cancer.
MP1 scaffold protein
MP1 (MEK partner 1) is a 14 KDa (124 amino acid) scaffold protein that is present in several species with various degrees of homology to humans: mouse (124 aa, 97% homology), rat (124 aa, 96%), frog (Xenopus laevis; 123 aa, 87%), fruit fly (Drosophila; 124 aa, 45%), and worm (Caenohrabditis elegans; 145 aa, 20%). The 16.2 kb MP1 gene is found on chromosome 4 and has seven exons and six introns. Alternative splicing results in three transcripts, with variant 1 being the longest transcript that encodes the 124 amino acid protein. According to NCBI RefSeq, transcript 3 encodes a shorter isoform (117 aa) but neither the CCDS (Consensus CoDing Sequence) database nor the UniProt database provide public data for this second isoform. MP1 was originally identified as a scaffold protein that specifically binds MEK1 and ERK1, and its ability to regulate MAPK signaling and its localization to late endosomes are the most well characterized functions of this protein.
The scaffolding role of MP1 has been demonstrated in several in vitro studies that report its association with large signaling complexes located to different intracellular compartments. For example, in late endosomes, MP1 interacts with adaptor proteins p14, p18 and KRas to induce ERK activation. p14 is required to localize MP1 to late endosomes, and although they have different primary sequences the two small proteins have similar structures and form a high affinity heterodimer. Biochemical and crystallographic analyses found that both proteins consist of a five-stranded β-sheet flanked by three α-helices. No specific protein interaction domains were identified. However, the structures of MP1, p14 and their heterodimer indicate the presence of several surface-exposed residues, suggesting the possibility of interaction with cytoplasmic proteins.
A more recent study reports the presence of MP1 in the mTOR pathway. In this study, the MP1/p14/p18 trimeric, coined Ragulator, interacts with and localizes Rag GTP-ases to lysosomes. Moreover, MP1 knockdown in HEK392T cells prevents the lysosomal recruitment of mTORC1 in the presence of amino acids and inhibits the amino acid-induced stimulation of dTORC1 in Drosophila, suggesting that MP1 is required for the recruitment of mTORC1 to lysosomes and is essential for the amino acid-dependent activation of the complex. MP1 is also required for PAK1 to activate a population of MEK1 in focal complexes during adhesion of rat fibroblasts to fibronectin and this interaction is sufficient to activate MEK1 in the absence of Raf, but to a lesser extent than observed with active Raf or EGF. Coimmunoprecipitations carried out in CCL39 mice fibroblasts shown an interaction between MP1 and RACK1, a scaffold protein that is required for the presence of active ERK to focal adhesions. In a family (humans) with an immunodeficiency syndrome, a mutation in 3' UTR for exon 4 lead to reduced ex