U.S. stock index futures fell on Tuesday after China, a main source of global growth, raised its interest rates, while Apple shares fell after the stock's weight in a widely followed index was cut.

China's central bank increased interest rates for the fourth time since October, raising suspicions that data next week may show inflation rose more than expected in March. Any move to curb overheating in China is seen as negative for global growth.

Also clouding the outlook for the global economy, Portugal's credit rating was downgraded again on Tuesday. The country's biggest banks threatened to stop buying government debt, according to local news reports.

Futures are being dragged lower by China raising its interest rates and a spike in concern over European weakness, said Oliver Pursche, president at Gary Goldberg Financial Services in Suffern, New York.

Apple Inc had its weighting cut in a rebalancing of shares in the Nasdaq 100, forcing some to sell the iPhone maker's stock. Once the rebalancing is effective on May 2, the projected weight of Apple will be 12.33 percent of the index compared with its current 20.49 percent, Nasdaq OMX said on its website.

The rebalancing requires sponsors of exchange traded funds which track the Nasdaq 100 <.NDX>, like the heavily traded PowerShares QQQ Trust , to also lower their holdings of Apple shares. Apple fell 1.5 percent in premarket trading.

In the short-term this is going to have a significant impact and create additional volatility, Pursche said.

We don't see it as long-term disruptive to the growth and performance of the individual securities.

Microsoft Corp's weighting in the index will rise, and its shares gained 1.7 percent before the open.

S&P 500 futures fell 3.2 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures lost 18 points and Nasdaq 100 futures dropped 16 points.

Other tech stocks in focus include Texas Instruments after it launched a $6.5 billion takeover bid for National Semiconductor Corp , offering a 78 percent premium. National Semi traded up 71.6 percent at $24.15 premarket.

Investors will scour the minutes from the latest meeting of the Federal Reserve's policy-setting committee. Fed Chairman Ben Bernanke said a recent rise in U.S. inflation was driven primarily by global commodity prices and was unlikely to persist.

Economic news includes the release of the Institute for Supply Management's March services sector index at 10 a.m. (1400 GMT). Economists in a Reuters survey forecast a reading of 59.5 versus 59.7 in February.

The S&P 500 met tough resistance on Monday, failing to break a level that has held since mid-February. It ended flat even as a spate of deals and underlying strength in the economy spurred optimism.

About 5.94 billion shares traded on the New York Stock Exchange, NYSE Amex and Nasdaq, the lowest total of the year.

(Reporting by Rodrigo Campos; Editing by Kenneth Barry)