NAB shares were the biggest mover among the banking stocks yesterday, falling 98c, or 3.57 per cent, to $26.47. While Australian banks are relatively immune to the problems driving the downturn in the US housing market, NAB is said to be the most exposed to subprime assets among the major local banks. Unease about the level of disclosure came as NAB joined St George Bank, the Commonwealth Bank and ANZ in raising interest rates for home loan borrowers in response to the credit turmoil, lifting its standard variable mortgage rate by 0.15 percentage points to 9.61 per cent. In its half-yearly report to March, NAB said it had provided liquidity lines to a number of "conduits" - or investment trusts that sit off the bank's balance sheet. While the bank is not directly exposed to assets held by the trusts, those trusts can call on the NAB if they are short of liquidity.