Table I. Common restraints that can be used for integrative structure determination.

Restraint	Description	Source of information
Excluded volume restrainta	Prevents steric clashes between system particles	Physical first principles
Geometric complementarity restrainta	Restrains a protein interface to the tightest possible packing	Physical first principles
Statistical potential restraint	Restrains a structure to have contact frequencies similar to those in structurally defined complexes	Physical first principals, all previously determined protein structures
Distance restrainta	Restrains the distance between two particles	FRET, BRET, cross-linking, homology to a known structure
Protein localization restraint	Restrains a protein to a specific position	Immuno-EM, gold labeling, GFP labeling
Protein connectivity restrainta	Restrains all proteins in a set to interact directly or indirectly	Affinity purification
Angle restraint	Restrains the angle between three particles	EM, SAXS, homology to a known structure
Complex diameter restraint	Restrains the distance between the two most distant particles in a protein or complex	EM, SAXS
Symmetry restraint	Maintains the same configuration of equivalent particles across multiple symmetry units	EM, SAXS, homology to a known structure
EM quality-of-fit restrainta	Restrains the model to overlap with a density map	EM, SAXS
Radial distribution function restraint	Restrains the correlation between experimentally measured and computed radial distribution functions	SAXS

^a Restraints used to determine the structure of H-RNAPII.