{{tag>beta}}
====== Greek Key =======

This is a four-stranded [[glossary:beta_sheet | beta sheet ]] [[glossary:motif | motif]] which is characterised by +3, -1, -1
connectivities in a two-dimensional schematic diagram of a protein structure, as shown here. The diagram on the left shows an N-type Greek key motif, whilst on the right is a C-type Greek key.

{{:glossary:greek_keyn.gif?400|}}{{:glossary:greek_keyc.gif?400|}}

In three dimensions, the motif adopts a variety of structures.
The Greek key can be considered as a chain folded in half, then twisted
around to form a pattern like that commonly seen on Greek vases.
Greek keys can occur with all four participating strands in the same beta sheet, or in two different sheets. A four-stranded Greek key motif can be defined by the following rules:
four sequential strands must be either in one, or at most, in two different beta sheets;
within any one sheet the participating strands must be hydrogen bonded together;  hydrogen-bonded strands within one sheet must be antiparallel; and strands 1 and 4 must be hydrogen bonded.

The example structure shown here is the beta/gamma crystallin domain (CATH domain ID 1prrA01). 

{{cath:feature>1prrA01}}

===== References =====

{{pubmed>long:7020376}}
{{pubmed>long:8506258}}