What does "parallel in-register" mean for a protein structure in a fibril? I read a paper by Ross and Poirier [1, p. S12] which has the following lines (see section Commonalities of amyloid structure) that describe the structure of protein in a fibril:


  The most extensively characterized amyloid fibril is that formed from the β-amyloid (Aβ) peptide implicated in AD. Using SOLID-STATE NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY, the in-register, parallel β-sheet organization of fibrils formed by Aβ10–35, a fragment of the full-length 42-residue Aβ peptide, was first described.


I couldn't understand what "in-register, parallel β-sheet" means.

References


Ross, C. A.; Poirier, M. A. Protein Aggregation and Neurodegenerative Disease. Nat Med 2004, 10 (7), S10–S17. https://doi.org/10/dppb95.
There is a clear explanation and figure showing this in the following reference:

Anton Gorkovskiy, Kent R. Thurber, Robert Tycko, Reed B. Wickner, PNAS 2014, 111 (43) E4615-E4622.

A parallel beta sheet is one where the direction from N- to C-termini on adjacent strands run in a parallel direction (rather than antiparallel). In-register means that each residue in one molecule is aligned with the same residue in the neighboring molecule, producing lines of identical residues along the long axis of the filament.