MED28 mRNA is identical to AF317679, AF358829, and AF321617 with the respective size of ~1, ~1.3, and ~3.2 kb (Wiederhold et al., 2004). They vary in the untranslated region, but share the same protein product.

The full-length MED28 protein consists of 178 amino acids with a predicted molecular weight ~20 kDa (Wiederhold et al., 2004). MED28, an evolutionaly conserved protein, was identified as an endothelial-derived gene, EG-1 (Liu et al., 2002). MED28 was also identified as a merlin-interacting protein by yeast two-hybrid screen (Wiederhold et al., 2004), where merlin, the neurofibromatosis 2 (NF2) tumor suppressor protein, belongs to the ezrin-radixin-moesin (ERM) family members. Affinity-binding assays demonstrated that MED28 interacted with the SH3 domains of Grb2 (Wiederhold et al., 2004). Accordingly, MED28 was also named as magicin, merlin and Grb2 interacting cytoskeletal protein (Wiederhold et al., 2004). In addition, several Src family members, including Src, Fyn, and Lck, are also MED28-associated partners (Lee et al., 2006; Lu et al., 2006). Therefore, MED28/magicin/EG-1 may function as an adaptor/scaffold to relay cellular signaling (Lee et al., 2006). Furthermore, MED28 is also found in the nucleus where it is presumably involved in gene transcription as part of the Mediator complex (Sato et al., 2004). It is likely that MED28 shuttles between nucleus, cytoskeleton, and cytoplasm to convey its physiological role.

MED28 is expressed in multiple human tissues (Wiederhold et al., 2004), with high expression in liver, placenta, and testis (Liu et al., 2002).
MED28-interacting proteins: merlin, actin, Grb2 (Wiederhold et al., 2004), Fyn, Lck, Src (Lee et al., 2006; Lu et al., 2006), Mediator subunits (Sato et al., 2004).

Cytoskeleton, cytoplasm, and nucleus (Sato et al., 2004; Wiederhold et al., 2004).

MED28 may be involved in the regulation of numerous biological processes, including transcriptional regulation, cytoskeletal organization, signal transduction, cell proliferation, differentiation, angiogenesis, and migration (Beyer et al., 2007; Huang et al., 2012; Liu et al., 2002; Wiederhold et al., 2004; Yoon et al., 2010; Zhang et al., 2004).

Transcription regulation:
The Mediator complex is a set of protein coactivators that bridges DNA-binding transcription factors to transcriptional activation of the RNA polymerase II (pol II). MED28 was identified as a subunit of the mammalian Mediator complex by the multidimensional protein identification technology (MudPIT) (Sato et al., 2004). This identification indicates a role of MED28 in the regulation of gene transcription.

Cytoskeletal organization:
The implication of MED28 in cytoskeletal re-organization was based on the observation that MED28, similar to merlin, co-localizes with the actin cytoskeleton as determined by cofractionation, immunofluorescence and electron microscopy (Wiederhold et al., 2004).

Signal transduction:
Ectopic expression of MED28 led to increased phosphorylation of mitogen-activated protein kinases (MAPK) (Lu et al., 2005). In contrast, MED28-specific knockdown resulted in decreased expression of mitogen-activated protein kinase kinase 1 (MAP2K1/MEK1) in MCF7 cells (Huang et al., 2012). In addition, overexpression of MED28 resulted in the activation of c-Src (Lu et al., 2006) and MED28 can be phosphorylated at Y64 by Fyn, a Src family member (Lee et al., 2006). These data indicate that MED28 may function as a regulator of cellular signal transduction pathways.

Cell proliferation:
Ectopic overexpression of MED28 resulted in enhanced cell growth in immortal human embryonic kidney (HEK) 293 cells, and breast cancer cell lines, MCF7 and MDA-MB-231 (Lu et al., 2005). The HEK293 xenograft tumor growth was also stimulated in MED28-overexpressing cells (Lu et al., 2005).

Smooth muscle cell differentiation:
MED28 functions a repressor of smooth muscle cell (SMC) differentiation (Beyer et al., 2007). MED28-specific knockdown resulted in up-regulation of several SMC-related genes and SMC phenotype, whereas ectopic expression of MED28 reversed the SMC morphology induced by MED28 knockdown (Beyer et al., 2007).

Angiogenesis:
MED28 expression was increased by two-fold in human umbilical vein endothelial cells (HUVECs) treated with either tumor conditioned media or specific angiogenic factors (Liu et al., 2002). This observation suggests a role of MED28 in angiogenesis.

Cell migration:
The role of MED28 in cell migration was demonstrated in both estrogen receptor (ER)-positive and negative human breast cancer cell lines (Huang et al., 2012; Lee et al., 2011). Ectopic expression of MED28 increased cell migration and invasion in MCF7 and MDA-MB-231 breast cancer cells (Huang et al., 2012; Lee et al., 2011). Investigation of the underlying mechanisms revealed that MED28 activates epidermal growth factor (EGF)-stimulated migration through induction of matrix metalloproteinase 9 (MMP9) in EGFR-overexpressing MDA-MB-231 cells (Lee et al., 2011). In addition, MED28 can also regulate cell migration through the mitogen-activated protein kinase kinase 1 (MAP2K1; MEK1)-dependent MMP2 activation, which is independent of EGF stimulation (Huang et al., 2012). These lines of evidence support the role of MED28 in cell migration, implying that MED28 might be responsible for cancer metastasis.

MED28 has significant homology to a mouse cDNA (94%), fish, and a fly cDNA (31%) (Liu et al., 2002; Wiederhold et al., 2004). The mouse MED28 gene is located on chromosome 5 (45520229..45529284).