Hierarchical assembly and the onset of banding in fibrous long spacing collagen revealed by atomic force microscopy

The mechanism of formation of fibrillar collagen with a banding periodicity much greater than the 67 nm of native collagen, i.e. the so-called fibrous long spacing (FLS) collagen, has been speculated upon, but has not been previously studied experimentally from a detailed structural perspective. In vitro, such fibrils, with banding periodicity of ∼270 nm, may be produced by dialysis of an acidic solution of type I collagen and α₁-acid glycoprotein against deionized water. FLS collagen assembly was investigated by visualization of assembly intermediates that were formed during the course of dialysis using atomic force microscopy. Below pH 4, thin, curly nonbanded fibrils were formed. When the dialysis solution reached ∼pH 4, thin, filamentous structures that showed protrusions spaced at ∼270 nm were seen. As the pH increased, these protofibrils appeared to associate loosely into larger fibrils with clear ∼270 nm banding which increased in diameter and compactness, such that by ∼pH 4.6, mature FLS collagen fibrils begin to be observed with increasing frequency. These results suggest that there are aspects of a stepwise process in the formation of FLS collagen, and that the banding pattern arises quite early and very specifically in this process. It is proposed that typical 4D-period staggered microfibril subunits assemble laterally with minimal stagger between adjacent fibrils. α₁-Acid glycoprotein presumably promotes this otherwise abnormal lateral assembly over native-type self-assembly. Cocoon-like fibrils, which are hundreds of nanometers in diameter and 10-20 μm in length, were found to coexist with mature FLS fibrils.