Abstract
SummaryHuman type 1 insulin-like growth factor receptor (IGF-1R) signals chiefly in response to the binding of insulin-like growth factor I. Relatively little is known about the role of insulin-like growth factor II signaling via IGF-1R, despite the affinity of insulin-like growth factor II for IGF-1R being within an order of magnitude of that of insulin-like growth factor I. Here, we describe the cryoelectron microscopy structure of insulin-like growth factor II bound to a leucine-zipper-stabilized IGF-1R ectodomain, determined in two conformations to a maximum average resolution of 3.2 Å. The two conformations differ in the relative separation of their respective points of membrane entry, and comparison with the structure of insulin-like growth factor I bound to IGF-1R reveals long-suspected differences in the way in which the critical C domain of the respective growth factors interact with IGF-1R.
Highlights
To begin to address this shortcoming, we present here single-particle cryoelectron microscopy (cryo-EM) structures of IGF-II bound to the intact ectodomain of IGF-1R
The2 form of IGF-1Rzip was produced by stable expression and secretion from CHO-K1 cells and purified by a combination of 9E10 antibody-affinity chromatography (Hoogenboom et al, 1991) and three sequential size-exclusion chromatography steps to remove4 forms of the zippered ectodomain wherein the leucine zipper forms between2 dimers rather than within2 dimers (Figures S1A–S1C)
We note that the IC50 values reported here for IGFs binding to holoIGF-1R differ from those reported by, for example, Machackovaet al. (2019) yet broadly concur with those reported earlier by Surinya et al (2008); the source of such variation is unclear but may relate to the use of whole-cell versus immunocapture assay formats
Summary
The human type 1 insulin-like growth factor receptor (IGF-1R; Figure 1A) is a disulfide-linked homodimeric member of the receptor tyrosine kinase family (Ullrich et al, 1986; Lemmon and Schlessinger, 2010) that signals into Ras/ERK or PI3K/Akt pathways in response to activation by the insulin-like growth factors I and II (IGF-I and IGF-II) (Adams et al, 2000; Denley et al, 2005; Riedemann and Macaulay, 2006; Laviola et al, 2007; Tao et al, 2007). The bioavailability of IGF-I and IGF-II is controlled by six insulin-like growth factor-binding proteins (Baxter, 2014), and IGF-II is sequestered by the membraneanchored type 2 insulin-like growth factor receptor (IGF-2R) that can influence signaling via G-protein interaction (ElShewy and Luttrell, 2009). The exon-11 minus isoform of IR (IR-A) can signal into growth and/or metabolic pathways in response to IGF-II binding (Belfiore et al, 2017; Holly et al, 2019), with the affinity of IGF-II for IR-A being only slightly weaker than its affinity for IGF-1R (Denley et al, 2005)
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