  While proteins have been treated as particles with a spherically symmetric
interaction, of course in reality the situation is rather more complex. A
simple step towards higher complexity is to treat the proteins as
non--spherical particles and that is the approach we pursue here. We
investigate the phase behavior of enhanced green fluorescent protein (eGFP)
under the addition of a non--adsorbing polymer, polyethylene glycol (PEG). From
small angle x-ray scattering we infer that the eGFP undergoes dimerization and
we treat the dimers as spherocylinders with aspect ratio $L/D-1 = 1.05$.
Despite the complex nature of the proteins, we find that the phase behaviour is
similar to that of hard spherocylinders with ideal polymer depletant,
exhibiting aggregation and, in a small region of the phase diagram,
crystallization. By comparing our measurements of the onset of aggregation with
predictions for hard colloids and ideal polymers [S.V. Savenko and M. Dijkstra,
J. Chem. Phys 124, 234902 (2006) and F. lo Verso et al., Phys. Rev. E 73,
061407 (2006)] we find good agreement, which suggests that the eGFP proteins
are consistent with hard spherocylinders and ideal polymer.
