What do a shadowy oil czar, a deeply religious railway boss and a billionaire judo enthusiast have in common?

They form most of what experts believe to be Vladimir Putin's inner circle — a coterie of old allies and confidants that defines the country's trademark system of crony capitalism.

Relying on Putin for their massive wealth and influence, they in turn shore up his power by controlling strategic sectors or resources.

Now that balance may be getting increasingly unsteady as the country veers toward an economic crisis — brought on by low oil prices and Western sanctions — that threatens to shake the stability Putin has long guaranteed.