The EMILIN/Multimerin family

Abstract

Elastin microfibrillar interface proteins (EMILINs) and Multimerins (EMILIN1, EMILIN2, Multimerin1, and Multimerin2) constitute a four member family that in addition to the shared C-terminus gC1q domain typical of the gC1q/TNF superfamily members contain a N-terminus unique cysteine-rich EMI domain. These glycoproteins are homotrimeric and assemble into high molecular weight multimers. They are predominantly expressed in the extracellular matrix and contribute to several cellular functions in part associated with the gC1q domain and in part not yet assigned nor linked to other specific regions of the sequence. Among the latter is the control of arterial blood pressure, the inhibition of Bacillus anthracis cell cytotoxicity, the promotion of cell death, the proangiogenic function, and a role in platelet hemostasis. The focus of this review is to highlight the multiplicity of functions and domains of the EMILIN/Multimerin family with a particular emphasis on the regulatory role played by the ligand-receptor interactions of the gC1q domain. EMILIN1 is the most extensively studied member both from the structural and functional point of view. The structure of the gC1q of EMILIN1 solved by NMR highlights unique characteristics compared to other gC1q domains: it shows a marked decrease of the contact surface of the trimeric assembly and while conserving the jelly-roll topology with two β-sheets of antiparallel strands it presents a nine-stranded β-sandwich fold instead of the usual 10-stranded fold. This is likely due to the insertion of nine residues that disrupt the ordered strand organization and forma a highly dynamic protruding loop. In this loop the residue E933 is the site of interaction between gC1q and the α4β1 and α9β1 integrins, and contrary to integrin occupancy that usually upregulates cell growth, when gC1q is ligated by the integrin the cells reduce their proliferative activity.

Keywords: EMI domain; cell migration; extracellular matrix; gC1q NMR solution structure; gC1q-dependent cell adhesion; skin homeostasis; α4β1 integrin.

Figure 1

Graphical view of EMILIN/Multimerin family members. Note that: (i) EMI domain: there is some experimental evidence suggesting that EMI domains might self-interact; however, since it is not clear if they form trimers they are depicted separated; (ii) Coiled-coil regions: the similarity between the central regions of the family members is only structural, the heptad repeats are placed in different positions along the primary sequence of the different members of the family. (iii) the so called “UNIQUE REGION” is represented in EMILIN1 by two leucine zippers followed by a functional 17 triplets long collagenic sequence; in EMILIN2 by a proline rich sequence followed by a non-functional 17 triplets long collagenic sequence; in MMRN1 by an arginine rich sequence; in MMRN2 by an EGF-like domain; (iv); the gC1q of EMILIN1 and EMILIN2 possess three unstructured loops (black triangles) each bearing a glutamic acid (E) able to interact with the α4β1 integrin.

Figure 2

Top: NMR solution structure of the homotrimeric EMILIN1 gC1q domain. The structure was downloaded from database of protein structures maintained at NCBI site (http://www.ncbi.nlm.nih.gov/Structure/mmdb/mmdbsrv.cgi?uid = 68072). Ribbon representation of the assembly, as side view, is presented in (A) the three protomers in the trimer are shown in different colors (pink, blue, and brown). Each monomer has a nine-stranded folding topology, with strands labeled according to the gC1q/tumor necrosis factor superfamily nomenclature. (B) The residues highly conserved in all the members of the gC1q superfamily and essential for a correct domain folding are shown in yellow only in one monomer for clarity. (C) Top view of EMILIN1 gC1q domain. The yellow bar highlights the solvent exposed position of the unstructured segment Tyr927–Gly945 (D) the X-ray structure of type VIII collagen gC1q, http://www.ncbi.nlm.nih.gov/Structure/mmdb/mmdbsrv.cgi?uid = 25284, showing the buried position of strand F. The same position of strand F is present in the X-ray solution structures of ACRP, type X collagen and Complement gC1q domains. Bottom: Sequence alignment between strand F (residues in bold) of representative members of the C1q superfamily and EMILINs gC1q. Asterisks indicate proteins for which the structure has been solved. Similar residues that are conserved in all proteins are shaded in gray, and the glutamic acid residue interacting whit α4β1 is in yellow.

Figure 3

The functions ascribed to the gC1q domain and to the other domains of EMILIN1 and EMILIN2 are summarized to show the versatility of these glycoproteins. The regulation of elastogenesis and of lymphangiogenesis have not yet been pinpointed to a specific domain although it is tempting to assume that gC1q plays an important role.

Figure 4

Elastin microfibrillar interface protein 1 in skin homeostasis. This illustration summarizes the proposed molecular mechanism underlying the regulatory role of EMILIN1 in skin. When EMILIN1 is expressed (left) TGF-β triggers cytostatic signals via pSmad2 (Ser465/467), down-modulates PI3K/Akt and hence PTEN expression is increased also because EMILIN1 binds to α4/α9β1 integrins. PTEN activation then inhibits the pro-proliferative activity of pErk1/2. When EMILIN1 is not present (right) the increased levels of mature TGF-β and the lack of α4/α9β1 integrin specific engagement downregulates PTEN determining the activation of the pro-proliferative pAkt and pErk1/2 pathways. In addition, TGF-β signaling is reduced since Erk1/2-phosphorylates the inhibitory Ser245/250/255 residues of Smad2.